US3552285A

US3552285A - Turret font photocomposing machine

Info

Publication number: US3552285A
Application number: US793622*A
Authority: US
Inventors: John M Mccall
Original assignee: Fairchild Camera and Instrument Corp
Current assignee: Fairchild Semiconductor Corp
Priority date: 1966-01-11
Filing date: 1968-06-18
Publication date: 1971-01-05
Anticipated expiration: 1988-01-05

Abstract

A photocomposing apparatus employing a variable-magnification lens system for projecting characters from a character font onto a record film at a variety of effective point-sizes, and in which positive and readily settable detent elements are provided to position the lens elements at the various precise positions required for the desired range of point sizes, by simple manual operations carried out in a few seconds for each such change. This is a division of Serial No. 519,906, filed Jan. 11, 1966 and allowed Mar. 18, 1968, now Pat. No. 3,434,402.

Description

United States Patent Inventor App]. No.

Filed Patented Assignee John M. McCall Huntington Station, NY.

June 18, 1968 Division of Ser. No. 519,906, Jan. ll, I966, Patent No. 5,434,402.

Jan. 5, 1971 Fairchild Camera and instrument Corporation Syosset, NX.

a corporation of Delaware TURRET FONT PHOTOCOMPOSING MACHINE 2 Claims, 9 Drawing Figs.

U.S. Cl. 95/4.5, 95/45 Int. Cl 1341b 15/34 [50]. Field of Search 95/45, 45

[56] References Cited UNITED STATES PATENTS 1,755,105 4/1930 Douglass 95/45 2,923,212 Corrado 95/45 Primary Examiner-John M. Horan Attorney-Homer R. Montague ABSTRACT: A photocomposing apparatus employing a variahie-magnification lens system for projecting characters from a character font onto a record film at a variety of effective point-sizes, and in which positive and readily settable detent elements are provided to position the lens elements at the various precise positions required for the desired range of point sizes, by simple manual operations carried out in a few seconds for each such change.

PATENTED JAN 5 I97! SHEET 1 BF 4 mvmon JOHN M. MCCALL,

ATTORNEY PATENTED JAN S|97| INVENTOR- JOHN M. Mam,

.commercial typesetting, will operate at a reasonably high speed, will permit rapid changing of the entire font to accommodate varying type face requirements, and will include provision for the change of magnification, by optical means, to vary the effective point size of the type face images laid down upon photographically sensitive record material.

Another object of the invention is to provide such a machine with a novel form of font element, specifically one in i the form of a small, hollow drum whose outer cylinder surface is concave or dished inwardly, and which carries the type face images on its outer face in rows and columns which are respectively lengthwise of the axial direction plane of the drum or spool, and in planes perpendicular to the drum axis. The concavity of the drum face provides for equalization of the distance to the face of a rotating mirror that is employed to select as amongst all the characters in a given ringlike column, and rotation of. the drum permits selection of the particular axial row desired. a

The outer, dished surface of the drum preferably is composed of a series of strip surfaces which are flat in the circumferential direction, and the cross section in a plane perpendicular to the drum axis is a polygon having as many sides as there'are character rows on the-drum. This ensures that the character images are not distorted by reason of the usual circumferential curvature of a cylindrical surface, and the radius of the dishing curvature is several times the diameter of the drum, so that only a tolerable alteration of character shape (which can be fully eliminated, if desired, by predistortion of an opposite sense) results.

The small size and low mass of the hollow drum font satisfies a further object of the invention, which is to minimize its inertia and hence minimize the time required for the drum to be positioned, in the rotary direction, by an intermittent mechanical selection drive, -to the end that selection of the needed character row can be accomplished in something like the same time that is required for the external selecting mirror to position itself at a chosen character image position in the axial-of-the-drum direction; and such thatboth these selection operations can be repeated at a speed commensurate with the requirements of commercial type composing.

An additional object of the invention is to free the equipment from the limitations of various known photocomposing schemes with respect to the need for an extremely high-speed flash of exposing light, the requirement being due to attempts to make the exposure while the font character, or the recording material, is actually in continuous motion. The present invention utilizes a short flash of light for each exposure, to obtain a high rate of repetition of character selections, but all of the fast-moving mechanical parts are at rest during such exposure, so that it can be comparatively long, of the order'of milliseconds.

Still another object of the invention is to provide apparatusof the above general type in which the final optical element in the train which images the character font images on the photo- 1 graphic film or paper is a' cyclically-swinging mirror or equivalent which sweeps across the exposure line area of the output record strip at a constant rate, to effect the sequence of placement of chosen characters with the desired spacing, including the spacing devoted to the width of the chosen character as well as the variable word-spacing utilized to effect line-width justification. As this deflector sweeps, and as chosen characters from the font are placed at the input end of the optical system, the exposure lamp, effectively inside the font drum, is flashed at the proper instants both to make the exposure and to make itat the proper instant for correct character placement. in order that the instants of flashing may be precisely related to the position of the rotating or sweeping deflector, a secondary optical system continuously monitors the position of the sweeping deflector with reference to a line grating positioned near the image recording (paper or film) end of the machine.

The invention also includes an unusually simple and lowcost optical system to permit selective magnification or pointsize control during the use of any given font element or font drum. This system utilizes a pair of selectively positionable projection lenses, in an essentially varifocal or zoom configuration, and devices to permit the operator to position them with precision at the necessary locations to provide properly focused type face images of specified point-size. The same optical system includes provisions for compensating for the slight variation in recording film distance (from the output mirror or deflector) as the beam sweeps along each composed line. Integrally related optical components provide for the maintenance of focus with respect to the grating-monitoring optics. Finally, the machine includes automatic provision for varying the line-composing'sweep rate in accordance with the pointsize selected by the setting of the optical system, to provide the greater line-composing time required when smaller point sizes are being set. In this connection, it will be recalled that the basic limitation on composing speed in this system is the rate at which the font drum can be repositioned for each successive character, so that a line of a given length requires more character selections per line and hence a longer line-composing interval.

Another object of the invention is to provide a machine of the above type which is especially adapted to the photo composition of type face images under the external selection control of a conventional punched paper tape record, or equivalent, such as the punched control tape of known signalcontrolled linecasting machines such as the Teletypesetter machine. Such tapes carry a record, in standard code, of the successive character selections, the word spaces, and various functional operations such as shift and unshift (or case selection), upper and lower rail selection, rubout (delete), end-ofline code, and so on. The present invention utilizes, when under the control of such a tape record, a special tape reader arrangement which first scans a one-line length segment of the tape to derive the information as to the aggregate of widths of all of the specified characters and word spaces for that line, and delivers this information to a computer which furnishes the necessary correlation by which the exposure lamp will be flashed when each character is in its proper projected position relative to the recording film or paper, to provide a justified line. Once this information has been derived from the tape, the latter is rapidly reversed to the starting position of its line record during the computing operation, and is read a second time to control the successive selections of the drum font and selecting mirror positions to effect actual character selection and projection.

The invention will now be described in detail, by way of illustration and explanation (and not for purposes of limitation), in connection with one preferred embodiment, reference being made to the accompanying drawings, in which:

FIG. 1 is a diagrammatic perspective view of the main physical components of the preferred embodiment of the apparatus.

FIG. 2 is an enlarged, fragmentary sectional view taken on line 2-2 of FIG. 1.

FIG. 3 is a diagrammatic perspective view of a tape reader arrangement for the apparatus.

FIG. 4 is an enlarged sectional view (compared to the scale of H6. 1) taken substantially along the line 4-4 of FIG. 1 but illustrating in more detail .the mounting of the font drum and its illuminating system.

FIG. 5 is a detailed, partly sectional view illustrating the construction of one of the position-sensing encoders, specifically the one that is associated with the selecting mirror drive.

FIG. 6 is a plari view of the optical system employed for counting grating spaces to control the location of exposure of on FIG. 4.

FIG. 8 is a sectional view taken along line 8-8 of FIG. 7. FIG. 9 is a simplified block diagram showing major components of the computing and control system typically employed with the invention, and their relation to the parts illustrated in the foregoing FIGS.

Referring to FIG. I. of the drawings, the major physical and :optical components of the photocomp'ojsing machine are shown as mounted upon a horizontal support 10 (of desk top size or smaller), with a conventional casing or cover removed for purposes of this illustration. The character font unit is indicated at numeral 12, its dished-drum or hour-glass conformation being indicated at 14 and providing a character row on each of its flat facets, the facets forming each arcuate row being centered at a point on the surface of the first-surface reflector 16 which point also lies in the vertical rotational axis I that-each lens carriage hasa

lateral rod

58, 60 which extends in contact with the grooved face of one wheel, thence beneath a locking bar. 62 pressed downwardly springs from a blocked upper limit position established fixed. stops (not shown); Where it passes beneath

bar 62Q'each rod

58 and 60 carries a cam such as 68, and both .rods extend forwardly to pass through a notched coarse-selection locating plate 70 hav, ing an upper longitudinal slot frornwvhieh depend individual vertical notches 72 correspqiidingtolthe gespective lens positions for a range oftype point sizes obtainablefrom-a particular 'font druml2. This, and from a typicahfgngdrum size of approximately 1% inch diameter and l kinch-length, with 24 equispaced facets and 9 characters along each facet, it is entirely feasible to provide a full range of exposure-plane type sizes such as 5-%6 ,,7, 8, 9, l0, l2 and 14 points. lb,

A locking positi'onof-

eachrod

58 and 60, in the rotary sense, is provided' by the cams 68, which urge the, rods downward when they are turned to raise the bar 62 against its upward stops. To change point size, both rods are rotated slightly to' lower bar 62 from its upper limited position, aIlow-- ing the rods to be raised out qfthe notches 72 and slid along the'upper slot in plate,,70 to a new position, then lowered (springs 64 and 66 urge them down) into the new notch 72, and again rotated to lock them in place. As the rods were raised, they left the V-grooves of the respective precise locat particular character from the selected axial row is positioned" along the. input axis of the following projection optical system. The stepper motor control includes a coded-spot digital transducer track on the rim of encoder drum 26, which allows the instantaneous stepper position to be sensed by a suitable array of photocells, or the like, indicated as a unit by numeral within the encoder drum 26, one axial row of code holes being illuminated by a lamp and condenser lens system 32. The details of the encoder drum optics are shown in FIG. 5 of the drawings.

The font drum 12 has an integral stub axle or quill which snaps removably into a socket in a shaft driven by an identical stepper motor 36, whose position'is also detented as at 37 and is readable by a digital coded-spot encoder drum transducer to identify the angular setting of the font drum in terms of the energization of the cell array represented at 38. The control systems of both stepper motors are related so as to provide in effect an X-Y coordinate selecting system for displaying in dividual font characters in succession to the projection optics that will be described below.

As also indicated in FIG. 1, but in more detailin FIGS. 7 and 8, the font drum is hollow, with the far end open to receive a relatively think, flat right-angle prism 40 that directs a concentrated, flat beam of light upon the rear surface of I each column of font characters, from a concentrated, highspeed xenon flash lamp 42 and the cylindrical condenser lens 39.The end faces of the prism are curved to act as lenses to provide in combination with the condenser lens, proper spread and concentration characteristics for the font-column illumination, as well as to neutralize the curvature of the transparent wall of the font drum insofar as it affects font illumination.

The image of each selected font character proceeds from selecting reflector 16 through a fixed objective lens 43 which forms a real image of the selected font character approximatelyat point 45, this image forming the real object of a system comprising the pair of

lenses

44, 46 which together form a variable-magnification system, being slidably mounted on a bearing rod 48 secured by bracket 49 to the underlying sup port 10. A second rod, 50, mounted parallel to rod 48, carries an' assembly of V-grooved wheels 52 which are individually located along rod 50 by means of nuts 54, 56 (for example) shown in FIG. 2 as threaded upon a threaded portion of rod 50.:The precise position of each V-groove wheel establishes one precise position of one of the carriages of

lenses

44 and 46, corresponding to a particular point-size change magnificatio ratio of the lens pair; this positioning' resu1ts from the fact ing wheels 52, and when again locked in place, they fall into a different wheel at the precise,=proper: position for the corresponding lens.= l-h'ere=is sufficient flexure in each rod 58 and 60-to ensure that the precisiomprovided by the close positioning of the'V-grooved wheels will not be overcome .by rough handling of the control knobs on the 'rods,' and approximate alignment of the point-size scale marlied fon plate 70 is adquate:-The new positions of the

lenses

44 and 46 provide -correct focus at'the' image'plane of the systeml with the specified and selected magnification, and without requiring displacement of either the filmplane or the font drum loca-' projection lens 78 "*i'nounted in a support frame 80. 'slidable through a limited range along the bearing rods, under control of a lever arm 82 pivoted at one end on the basic support 10 and having its other end spring-urged against a cam 84 on a shaft 86 connected by a magnetic clutch 88 to gearing 90 driven by the sweep-control motor 92. The operating coil for the clutch is indicated at 94.

The character-bearn leaving lens 78 is reflected at a firstsurface reflector 96 whose face lies in the vertical plane of rotation of a shaft 98 journaled ih the basic support of the ap- [position of the next'line commences.

paratus, this shaft being secured to a lever 99.which is springurged against a sweep cam 100 also onthe shaft 86that is controlled by'clutch 88.180 long as the clutch is engaged, the cam 100 causes a relatively slow traverse of theoutput'beam from Inorden-to maintain the character beamarriving at the record 'film'iin proper focus despite the fa ct that the distance from the axis of the reflector 96 to the record film is slightly greater when the beam is near either edge of the line, the cam 84 is given a heart-shape profile of such configuration as to move the final lens 78 just sufficient, during each sweep, as to compensate for the slight change in imaging distance. This provision eliminates the need for providing means for curving the record film in itstransverse direction, as would otherwise be required to prevent serious defocusing either at the film center or bothedges.

From what has been said above, it is obvious that the correct positioning of each successive character, along the composing line area 102, requires that the flash lamp 42 be fired after the desired character has been placed on the optical projection axis, by

motors

24 and 36, and at precisely the correct instant during the sweeping progress of the beam direction leaving mirror 96. Since the lamp will be flashed only intermittently, as each selected character image arrives at the correct point along the composing line, it is preferable to think of the beam direction defined by mirror 96 as traveling along the line 102, or to think of the sweeping of a potential image" of the selected character along a portion of the line, as there will not be a useful optical image (nor, indeed, any light at all) projected onto the film 104 except when the lamp is fired. The correct position at .which the beam direction should lie, when the lamp is fired, is determined (1) by the sequence of character widths called for since the beginning of that line, and (2) by the aggregate of the word spaces including any increments to such word spaces dictated by the necessity for line justification.

To establish the precise position of the sweep at which the lamp is fired, the'sweep is monitored by an optical system including the constantly illuminated light source 108 and condenser 110 which illuminates a narrow masked slit at 112. These parts are disposed slightly above the geometrical axis defined by lenses 11 6 and 118, for a reason to be described. The light from slit 112 passes to the fixed focusing lens 116 in support 76, a movable focusing lens 118 in support 80, and a first-surface reflector 120 mounted on shaft 98 parallel to reflector 96 already described. The monitor beam is thus scanned in a horizontal plane in the same manner as the projection beam for character imaging, but at a location above the recording film 104, so that the beam traverses an optical grating 126, preferably having a large number of alternate opaque and transmitting line areas, so-that as the monitor beam scans the grating, a number of pulses will be produced that exactlyv define the distance of the monitor beam, and hence the composing beam, from a zero position at the starting end of the line format.

The grating effectively covers one longitudinal half of the hypotenuse face of a totally-reflecting prism block 124, which returns the (now) pulsed or grating-chopped beam generally in the same direction from which it arrived from reflector 120, but not precisely so, as explained in the next paragraph. Between the grating and the prism block is placed a rectangular-profiled center-slice 122 of a planoconvex lens, to collimate the beam from lens 118 and decollimate it after it has been redirected by the prism. 1

It will be recalled that slit 112, forming the object of this portion of the system, was positioned above the geometrical axis of

lenses

116 and 118. Hence, the image rays from slit 112 will arrive at prism block 124 below the geometrical axis, and after total reflection within the prism (whose face height is centered on the geometrical axis), will start their return path at a location above the axis. Crossing the axis between

lenses

118 and 116 during their return, the light rays strike a reflector 114 positioned below the geometrical axis, and by it the light is directed laterally to a condenser lens 127 and a counting photocell 128. The latter is connected to control, ultimately, the instant during the sweeping action at which each character in succession is exposed on the recording film 104. The crossover optical path just described is detailed in FIG. 6, simplified by the elimination of the beam deflection due to reflector 120.

As already indicated, the input to the photocomposing machine is preferably derived from preprepared punched tape of the Teletypesetter type, well known to those skilled in the art. To avoid complicating FIG. 1 with the details of the input information paths, these are indicated in FIG. 3 of the .ward direction beneath a preferably photoelectric codesensing head 138. A rapid reverse motion of the tape, back to the beginning of a series of line-composition codes, as sensed by head 138 or otherwise, is obtained by reverse-drive sprocket 140 also powered from the clutch mechanism 134, so arranged that when either sprocket is being driven, the other sprocket is allowed to idle. To permit rapid reverse motion of the tape without the need to drive the takeup reel system 142 against the takeup torque, a substantial loop of tape at 144 is allowed to accumulate during the forward motion, by proper adjustment of the takeup speed of reel 142 to fit the average tape-takeup speed required in operation.

As already mentioned, and as will be described in more detail in connection with FIG. 9, the apparatus is cycled so that, at the commencement of a line-comprising cycle, the tape 130 is first scanned or sensed throughout one line-composing segment of its length, the head output being passed to a computer 146 which decodes the character codes, calculates the needed width of composing line space for each, registers these and their totals, adds the necessary allowance for word spaces, subtracts such total line-width requirement from the preassigned length desired for the line, and allocates any line shortage as nearly equally as may be amongst the word spaces for justification. It is noted that the facility for rereading the perforated tape, before actual photocomposition commences, makes it unnecessary for the computer to memorize the actual sequence of character codes, because these will be furnished to it during the second part of the tape-reading cycle. A sharp reduction in memory requirement in the computer can thus be achieved, but it is to be noted that the present invention is not limited to such a double reading of each line section of the control tape, and can in fact be directly controlled by a manual keyboard, if adequate memory capacity is provided in the computer, and if the keyboard is furnished with the usual linefilling indicators, as employed in Teletypesetter keyboarding practice.

The outputs of the computer 146 are indicated in FIG. 3 at 148 and 150, for the X and Y coordinate positions of the successive characters, and at 152 for the timing of the flash exposure during the sweep of the beam across the receiving film or paper. Another output, indicated at 154, provides the control for the reversing clutch control mechanism 134.

A very important and novel feature of the invention lies in the construction and arrangement of parts associated with the font drum 12, and these parts are accordingly shown to a larger scale in FIG. 4. The drum 12 is shown as one-piece casting or molding of glass, plastic or other transparent material, having one closed end connected to a stub shaft or quill 156 which scraps into a bore in shaft 161, carried by a precision ball bearing 158 carried by a suitable bracket 160 on basic support 10. A flat on the quill ensures approximately correct positioning of the font drum with reference to the shaft 161, when the quill is inserted, but precise angular positioning is achieved by engagement of a pin 162 on the hub 163 of the shaft 161, in a registry slot in the end face of the font element. Shaft 161 also carries the electromagnetic detent wheel 37 or equivalent detenting means which ensures that the selected facet of the font drum is in position for scanning by the selecting mirror 16 (FIG. 1), and the shaft terminates in an Oldham coupling 165 which connects it to the shaft of stepper motor The facets of drum 12 are flattened (circumferentially) to avoid distortion of the character images; only one line of I characters is indicated in FlG 4, for clarity. These may be To the left of the drum 12, FIG. 4 illustrates the xenon flash lamp 42 in the lamp house 166, the light from the lamp being collected bya cylindrical condenser lens strip 168 on to the lenticular face of the prism block 40 which is suitably carried by the forward lamp house wall 170, so that when the lamp house is retracted to the left by sliding along the fixed guides such as 172, ample axial space is provided for the removal of the font drum quill from socket hub 163, when a different font is to be substituted. In a typical case, the font drum, of dimensions given above, will carry 24 facets around its periphery, and 9 characters on each facet, providing an ample font selection of 216 characters.

' The block diagram of FIG. 9 shows in simplified fashion the control paths of signals developed both during the initial reading of each tape section, during which the proper word-space width is determined so as to produce a justified line, and'also during the second reading of the same tape section (after it has been returned idly to the starting code position) at which time the actual exposure of properly spaced letters and characters is achieved. The initial reading can be accomplished at very high speed, since these signals are being processed entirely by high speed computer circuits without corresponding operation of mechanical parts; thus, the tape may be read at a speed of 300 code groups per second or greater.

During the initial reading of each line section of the tape by the bidirectional tape reader 180 of FIG. 3, which is given the same reference numeral in FIG. 9, each plural-bit character code in turn is stored in the shift register 182. The shift register acts as a buffer for the interim storage of codes and their sequential delivery to the following circuits. Thus, it may provide forthe storage of several complete code groups, so that a code group can be read out of the register at the proper time, while another code group is being received therein from the tape reader. As each group is read out of the register 182 in turn to the decoder 184, which, during this initial cycle, recognizes the character (or space) called for by the code group, a corresponding signal is sent to the width encoder 186. Over path 188, all of these width-indicating'signals go to the word space computer 190 which adds up the line-space units required by all of the characters-to appear in that composed line, subtracts the total from the standard line-length figure; and divides the remainder amongst the number of word spaces that have been registered. The value of the word space that must be provided after each word in that line is thus arrived at, and is stored in the word space register 192. When this has been accomplished during the initial tape segment reading, the tape is returned automatically to the first code of the same line ready for a second reading during which photographic exposure ofcharacters onto the film will occur.

In this simplified description, no account is taken of the possibility that the space in a line required to be taken up by the word spaces may not come out even. The provision for one or more unequal-width word spaces can readily be introduced in ways that will be obvious, if such a refinement is included.

The second reading of the tape section will ordinarily occur at a somewhat slower speed, to accommodate the speed of the mechanical parts described above in connection with FIG. 1. Thesuccessive code groups are again buffered through the shift register 182 into the decoder 184, but the latter now also sends a character-identifying signal to the X-Y positionenl2 and mirror 16 has brought these parts to the proper setting coder 194, which translates the signal into the two'coordinat'es (font drum rotational position and selecting-mirror position) needed to control the presentation of the called-for character to the projection system of FIG. 1. Thus, the encoder sets comparator X to a digital value corresponding to the proper position of selecting mirror 16 (FIG. 1) while the comparator Y is set to a value corresponding to'the proper position of font drumlZ.

Simultaneously with the commencement of this. second reading of the'coded tape by the reader the clutch magnet 94 of the drive for the sweep mirror'964of- FIG. I has been engaged, starting one continuous sweep of that mirror across the composing line 102. This also initiatesia sweep of the mirror 120, so that photocell 128 commences to generate pulses, one for each of the spaces of grating 126. There area large number of spaces per each inch of grating length, for example a thousand or more, so that (depending upon the point size being composed) there will be thirty or forty or more in each typical character width. As the grating is scanned, the count of pulses since the previous character exposure (indicating the instantaneous position of the character beam direction along the line direction) is stored in counter 196 and presented to the comparator 198. I

The registration of beam position in comparator'198, obtained as just described, is constantly being compared with the registration therein of the'character-width last called for by the width encoder 186,'obtained from the OR circuit 2001 As the digital registration of beam position in comparator 198 is increasing, the previously described setting of the font=drum then reset to zero, and shift register 182 operated to supply'the' next character code to the decoder 184.

Since no exposure is required to compose a word space, the precalculated word spacing stored in register 192 is simply added, by the OR circuit 200, to the width registration in comparator 198 whenever a word space code is recognized in decoder 184. Thus, the following character is exposed on the film at the appropriately spaced position from thepreceding character.

There is a stepper drive circuit 204 for the selecting mirror drive indicated by the box labeled 22, 20, and a similarstepper drive circuit 206 for the font drum drive indicated 36. When the respective steppers are energized (by aychannel not shown) under the control of the X, Y position encoder 194, they operate the stepping motors until comparators X and Y signal that the selecting mirror and font drum have arrived at the required positions to expose the correct next character, and the stepper drives are then stopped. Connections 208 and 210 from the flash control circuit 202 are indicated to illustrate that these stepper drive circuits are again energized, after each character exposure, assuming the next character called' for is a different one. The detailed logical circuitry by which the various possibilities are implemented, such as the feedingout of the next character code from shift register 182, and optionally the driving of the selecting mirror and font in the shortest path (selective direction of rotation) to present the next character, have been omitted from FIG. 9, as unnecessary to toning of'the character row portions, equivalent constancy of focus could be achieved bymeans of a system including an optical field flattener or like compensating element to allow for the change in object-distance for different positions along the row. Also, with such a cylindrical font element, constancy of focus could be achieved by the use of a movable lens in the following optical system, controlled as to position by the rotations of the mirror. Thus, a rotating selecting mirror 16 is not an indispensable feature of the combination.

At the output end of the overall projection axis, the

tor 96 is referred to as cyclical by which is meant the repeated sweeping of the reflector to generate successive composed lines of characters on-the' recording film. lt will be realized, by

. those skilled in the art, that this term is not meant to imply that the successive sweeps are of equal length, because in the composition of lines of difi'erent lengths, the length of sweep will vary, as well as the overall repetition rate as between successive long-and short sweeps.

While the invention has bee n des cribed herein in considerable detail, to enable those skilled in the art to practice the same, various modifications in the arrangements will be seen to be possible, and accordingly the invention is not to be taken as limited to the described forms, except as may be required by the scope of the appended claims.

lclaim:

1. In a photocomposing machine of the kind including a variable magnification optical system for imaging character representations on a photographic medium, said optical system comprising a pair of axially-adjustable lens elements forming a variable magnification system, the improvement which comprises precisely adjustable position-detenting means for said lens elements, said position-detenting means comprising a threaded guide positioned parallel to the common axis of said lens elements, V-grooved positioning detents on said threaded guide, individual clamping nuts threaded on said rod to secure each of said detents in a precisely adjusted position thereon, a positioning element on each of said lens elements for selective engagement with a respective one of said detents, and means for releasably urging each of said positioning elements into engagement with one of said detents.

2. In a photocomposing machine as described in claim 1, means for selectively locking each of said positioning elements in engagement with a selected one of said detents.