WO1991000541A1

WO1991000541A1 - Alignment eyeglasses

Info

Publication number: WO1991000541A1
Application number: PCT/US1990/003098
Authority: WO
Inventors: J. Glenn Peters; J. Ben Crawford
Original assignee: Peters J Glenn; Crawford J Ben
Priority date: 1989-06-13
Filing date: 1990-06-08
Publication date: 1991-01-10

Abstract

Eyeglasses (10) to aid a wearer, especially athletes or participants in sports to diminish the input from his or her occulo-vestibular reflexes and provide immediate visual feedback concerning alignment while he or she is performing any motor coordinated skill or movement. More particularly, the eyeglasses (10) have one or more substantially straight visible transparent lines (22, 28) on one or both of the eyepieces (18, 20) which appear to be superimposed on the view through the line or lines. Each line is substantially parallel to the horizontal (H) or vertical (V) axis of its eyepiece (18, 20). This line or lines (22, 28) enable the wearer of the eyeglasses (10) to readily check alignment of the body, head, hands and/or a held object with a viewed object, e.g. a golf ball, a pitched baseball, a bowling pin, a basketball, etc. and aid the wearer to make alignment corrections if necessary.

Description

ALIGNMENT EYEGLASSES

Field of the Invention

This invention relates to eyeglasses having left and right eyepieces that are equipped with indicia especially adapted to improve, for a wearer, the results of a motor coordination skill, that is, to aid the wearer in making correct alignments of head, body, arms or legs or of held implements, such as, for example, a basketball, etc.. Objects of the Invention

Persons performing tasks resulting in eye, head, or body movements will experience normal occulo- vestibular reflex input which will affect any ballistic movement or judgment of movement. Well trained athletes can learn to suppress these reflexes to perform practiced maneuvers or ballistic movements, such as hitting a basketball, striking a golf ball, free throwing a basketball, a bowling ball, throwing a bowling ball, running, etc. It is a general object of this invention to diminish the input from the occulo-vestibular reflexes of the athlete or sportsman and provide immediate visual feedback concerning alignment while he or she is performing any motor coordination skill or movement. This is accomplished by providing the wearer of eyeglasses embodying this invention with immediate visual feedback which through repetitive performance or practice will train the wearer to either keep his or her head and eyes still or limit linear and/or rotational movement to one plane. The alignment eyeglasses will allow the wearer to keep the target or target line in the same position in the wearer's visual field for each repetition. Any movement in the visual field or of the visual field will be instantly apparent to the wearer. This allows more precise performance of the designated task. Eyeglasses embodying this invention are especially adapted for use by participants or trainees in various sports, such as golf, basketball, baseball, bowling, running, baseball umpiring, tennis linesman, etc. , which require correct alignment of the body, head, limbs and/or a held article, e.g. a basketball, a bowling ball, etc..

The eyeglasses may be especially designed for use by golfers to provide a visual reference line for the purpose of keeping the head in proper position for the golf swing and providing alignment information in preparation for the shot.

It is important for a golfer to keep the head steady or as immobile as possible during the swing, both backward and forward. An object of this invention is to enable a golfer to observe whether or not the head is steady and if not to make appropriate corrections.

Further, golfers sometimes when in position to strike the ball, i.e. at address, are not correctly aligned with the target, i.e. the direction in which the golfer desires the ball to go. An object of this invention is to allow the golfer to easily check that alignment and to make adjustments, i.e. reposition the feet if necessary, to achieve correct target alignment.

Still further, golfers at address sometimes are not correctly or desirably positioned as respects the location of the ball between the golfer's feet.

Sometimes the ball is positioned too far ahead, i.e. toward the target, or too far back. An object of this invention is to allow the golfer to easily check that position of the ball and to make an adjustment, i.e. reposition the feet if necessary, to achieve the desired location or position of the ball in the golfer's stance.

The eyeglasses may also be especially designed for use by basketball players during a free throw to provide a reference point to focus the wearer's attention prior to shooting or throwing the ball and to provide directional alignment for the player's head and body during the shot. These eyeglasses also can be used by a participant in any sport requiring the throwing of an object, e.g. a baseball, a football, etc. , from a stationary position.

The eyeglasses may also be especially designed for use by baseball batters to properly position the head when attempting to hit a pitched ball. The eyeglasses will keep the batter's eyes in the proper alignment and help provide immediate feedback when he moves his head away from the plate or strike-zone.

The glasses may be especially designed for use by bowlers to provide directional information to the bowler as he or she approaches the line across the bowling lane to release the ball.

The eyeglasses may also be especially designed for use by runners to show proper positioning of the runners head, help the runner move in a straight path between two points, and provide immediate feedback to help the runner keep his head and body from having excess movement. The eyeglasses will also provide a focal line to concentrate upon and aid in movement of arms, legs and feet in a straight path.

The eyeglasses may also be especially designed for baseball umpires to help the umpire visualize the strike zone. The eyeglasses may also be especially designed for a tennis linesman to help the linesman focus his attention on the line that he is responsible to judge.

Other uses of the eyeglasses embodying this invention will become apparent to those skilled in the art.

Brief Summary of the Invention The aforesaid objects are accomplished by providing a pair of eyeglasses of any conventional type, even clip-ons, having left and right transparent eyepieces on either or both of which there are one or more substantially straight visible transparent lines that provide the illusion of the lines being superimposed on the view through the eyepieces.

Depending on the particular usage of the eyeglasses, the line or lines may be parallel to and displaced from or pass through the horizontal and/or the vertical axis of the eyepiece. The horizontal axis of an eyepiece may be defined on an imaginary line extending horizontally, that is when the eyeglasses are worn by a person with the head in an upright position, through the center of vision or visual axis of the eyepiece. The vertical axis of an eyepiece also may be defined at an imaginary line extending vertically through the center of vision or visual axis of the eyepiece when the eyeglasses are worn by a person with the head upright. The lines preferably are continuous but may be discontinuous, such as dash or dot-dash, so long as the objective or objectives are achieved. They also desirably extend from side to side or from top to bottom of the_ eyepiece, but may be somewhat shorter so long as the foregoing objective or objectives are achieved. The lines preferably also are of measurable width and transparent while the areas of each eyepiece unoccupied by the lines also are transparent and uncluttered by any distracting indicia or other obstructions to vision. Other uses and objectives of this invention will become apparent from the following detailed description and accompanying drawings in which: Brief Description of the Drawings Figure 1 is a perspective rear view of eyeglasses especially designed for use by golfers having left and right eyepieces provided with visible lines thereon embodying this invention.

Figure 2 is a rear view corresponding only to the eyepieces of Figure 1 but showing a modification of the invention for use by golfers.

Figure 3 is a view corresponding only to the eyepieces of Figure 1 showing another modification of the invention especially designed for use by golfers.

Figure 4 is a view corresponding only to the eyepieces of Figure 2 showing a further modification of the invention especially designed for use by golfers when putting. Figure 5 is a view corresponding only to the eyepieces of Figure 2 showing a modification of the invention especially adapted for use by basketball players and runners. Figure 6 is a view corresponding only to the eyepieces of Figure 2 showing a modification of the invention especially adapted for use by bowlers. This modification also can be used by a tennis linesman.

Figure 7 is a view corresponding only to the eyepieces of Figure 2 showing a modification of the invention especially adapted for use by baseball batters.

Figure 8 is a view corresponding only to the eyepieces of Figure 2 showing a modification of the invention especially designed for use by a baseball umpire.

Figure 9 is a view corresponding only to the eyepieces of Figure 1 showing a modification of the invention especially adapted for use by typists. Detailed Description of the Invention

Referring now to Figure 1 of the drawings, there is shown eyeglasses 10 having the usual frame 12 with a nose piece 14 and temples 16. Held in the frame 12 are left and right transparent eyepieces 18 and 20 which may be corrective, i.e. prescription lenses, or non-corrective, i.e. plain lenses. The eyepieces 18, 20 may be formed of any suitable substantially transparent material such as glass, plastic, polycarbonate or the like. The frame 12 is shown for illustrative purposes only and the eyepieces 18, 20 may be held by any type of eyeglasses arrangement including that which can be clipped onto conventional eyeglasses. The eyeglasses 10 shown in Figure 1 is especially designed for use by golfers. Thus, in the left eyepiece 18, preferably displaced below but parallel to the horizontal axis thereof, indicated by the line H-H, there is a straight visible transparent line 22 which preferably extends from side to side of the eyepiece.

The visible line 22 is intended to be visible to the golfer when looking therethrough so that the line appears to be superimposed on the view through the left eyepiece 18. The visible line 22, or any of such other comparable lines described hereinafter, may be formed in any number of ways. If the eyepiece 18 has a base of clear material, e.g. glass, and is tinted by a tinted layer or layers or covered by a tinted surface coating on one or both surfaces of the eyepiece, the line 22 may be formed by a laser beam which cuts a shallow groove through the tinted layer or layer or tinted coating or coatings and exposes clear glass or other clear material therebeneath, as shown in Figure 1. The visible line 22 also may be formed by placing a masking strip, corresponding to the line 22, over the eyepiece 18 before the eyepiece is tinted with tinted layers or surface coatings and after it has been tinted the masking strip 11 is removed to disclose clear material therebeneath. The line 22 may also be lightly tinted, e.g. yellow, which may draw the eye more readily thereto. If the eyepiece 18 is clear to begin with, the masking may be applied in two parts to leave a visible line therebetween which then is shaded in the tinting process. hen the masks are removed, there is simply a tinted or shaded line 24 between two clear parts, as shown in Figure 2. Other methods of making the visible line will be apparent to those skilled in the art.

Desirably, the visible line 22, or any other comparable visible lines descibed hereinafter, is of measurable width and preferably may vary from one to two millimeters in width depending to some extent upon the illusion to be produced, i.e. the appearance of the line when superimposed on the view therethrough observed by the person wearing the eyeglasses. The width of the visible lines may be even greater, however, depending on a specific usage of the eyeglasses as described hereinafter. The principal object to be achieved is the appearance of the visible line which, according to this invention, is formed by at least one contrasting shade and is transparent. It is undesirable, however, that the visible line, however formed or defined, be opaque and so wide that it obscures an object to be viewed through the line, e.g. a golf ball.

Whether the major areas of the eyepieces 18 and 20 are tinted or darkened as shown in Figure 1 or clear as shown in Figure 2 will depend on the intended usage of the eyeglasses embodying this invention. If the user normally wears dark glasses or sunglasses for a particular activity at a particular location, then the major areas of the eyepieces will be tinted or dark, and vice-versa.

The displacement of the line 22 below the horizontal axis of the eyepiece 18 may be varied somewhat depending usually upon the height of the golfer. A relatively short golfer need not incline his head very much at address to look directly at the golf ball (not shown) , while a rather tall golfer usually must incline his head to a greater extent at address to look directly at the golf ball.

After addressing the ball and looking directly thereat through the visual axis of the eyepiece 18, the golfer then raises his head so that the line 22 apparently passes directly through the golf ball and along the ground. This indicates the direction in which the ball will go when it is struck by the golfer. That extended apparent line 22 thus indicates to the golfer whether the line is pointing in the correct direction, i.e. at the target. If not, the golfer makes necessary adjustments to properly align his feet so that the line 22 does point or extend in the necessary or intended target direction. In a somewhat reverse procedure, the golfer initially, by head movement, may align the line 22 with the ball and target direction and then adjust his feet to maintain that correct target direction during the swing. Referring again to Figure 1 of the drawings, on the right eyepiece 20 there is a straight visible transparent line 28 which is parallel to the vertical axis of the eyepiece, indicated by the line V-V, displaced from that axis toward the nasal side of the eyepiece and preferably extends from top to bottom thereof. Of course, the vertical line 28 on the eyepiece 20 of this invention may be formed instead like the vertical line 30 in Figure 2, i.e. in the same manner as the horizontal line 24, as a tinted or shaded line between two clear sections. The vertical lines 28 and 30 may be formed in any of the ways described above. The vertical visible line 28 or 30 is also used by the golfer by slightly turning his head to the right at address so that it passes directly through the ball. This will aid the golfer in determining where the ball lies with respect to the distance, i.e. along the line, between his feet. If the ball is either ahead or behind the desired position, the golfer can make the necessary adjustments, i.e. with his feet, to achieve the desired position of the ball in the golfer's stance. This vertical line 28 or 30 also aids the golfer in checking whether or not he is in a correct upright or substantially vertical position at address so that his body is not canted excessively to one side or the other. Still further, the line 28 or 30 will indicate to the golfer during the swing whether or not the head has moved so that if necessary the golfer can correct or eliminate such movement and make a more determined effort to keep the head still, a very important factor in striking the ball squarely.

Attempts have been made heretofore to provide eyeglasses that will aid a golfer in keeping the head steady during the swing. Thus, the patent to McMurdo 2,045,399 discloses tinted or colored eyeglasses having in the left or both eyepieces a clear segment in the left hand side of the eyepiece. The instruction is that at address the golfer sees the ball through the colored area of the eyepiece, then he turns his head to the right before commencing the swing, and the ball appears in the clear space or spaces and should remain there during the swing, provided the golfer keeps his head still. Although the clear spaces are defined by one or more straight lines, i.e. the boundary between the colored and clear areas, these are not used specifically as focus points to keep the head steady. Aside from the fact that the McMurdo lines have no measurable width, they mostly are so short that they cannot serve the alignment functions of the lines of this invention. In only one McMurdo embodiment (Fig. 2) is there defined a straight vertical line long enough to serve an alignment function. But this line is in the left eyepiece for the non-dominant eye and apparently passes through the visual axis. Thus, this line alone is of small use for a golfer. Moreover, the clear area 15 of McMurdo Fig. 2 is so large, in comparison with the size of the golf ball viewed therethrough, that the clear area is of little use in aiding a golfer to keep the head steady.

Another patent, to Novack 3,268,228 discloses eyeglasses purportedly of use to a golfer in keeping the head still during the swing. Novack, like McMurdo Fig. 2 shows small clear segments in both eyepieces, but the remaining areas of the eyepieces are opaque so the eyeglasses must be removed after the swing to enable the golfer to see sufficiently to be comfortably mobile. Still another patent to Underhill 3,871,104 discloses eyeglasses having cross-hairs in both eyepieces which, however, are hollow and contain a non-transparent liquid the upper level of which registers precisely with the horizontal cross-hair of the corresponding eyepiece when that cross-hair is horizontal. The eyeglasses are intended to be a substitute for two tools, i.e. the usual carpenter's bubble level for horizontal leveling and a plumb bob to determine vertical lines. These eyeglasses would be of little use to a golfer.

After the aforesaid correct address has been achieved by use of the horizontal and vertical lines the golfer then lowers his head and looks directly at the ball while keeping the line 28 or 30 aligned with the ball and the golfer is in a correct position and correct alignment to strike the ball so it will go in the proper direction and achieve the desired loft because the golfer has placed it correctly as respects his two feet, that is, the distance between the two feet. Further, during the swing, the golfer can more readily keep the head still because of the feedback from the vertical line 28 or 30 which should pass directly through the ball.

Preferably, the angle between the visible lines 22, 28 on the left and right eyepieces 18, 20 is of the order of 90°. The exact location of the visible lines on the eyepieces 18, 20 may vary on different eyeglasses because of differences in pupillary distance and individual preference for where the visual illusion of the lines on the view is desired. While the visible horizontal line 22 preferably is displaced below the horizontal axis H-H and the visible vertical line 28 is displaced from the vertical axis V-V toward the nasal side, such displacement is not essential for some ^c of the functions of the visible lines, i.e. all functions but that of aiding the golfer in keeping his head still. On the other hand, when the eyeglasses 10 are worn continually when a golfer is playing, as normally would be the case with sunglasses or prescription glasses, locating the visible lines at or near the visual axes of the respective eyepieces would place the lines in position to be viewed continually. That probably would be distracting. Accordingly, the above-described displacement from the axes is preferably for golfers or any other activity requiring proper alignment when the glasses are worn continually.

The vertical line 28 is intended for use by the dominant eye of the golfer. If the right eye is dominant the foregoing description is apt for a right handed golfer. However, if the left eye is dominant and the golfer is left handed the positioning of the visible lines 22, 28 or 24, 30 will be reversed with respect to the eyepieces 18 and 20. Referring now to Figure 3 of the drawings, there is shown a further modification of the Figure 1 embodiment especially designed for golfers wherein there are two visible horizontal lines 32, 34 on the left eyepiece 18, both displaced from each other and both preferably displaced below the horizontal axis of the eyepiece. This type of modification can be used to provide eyeglasses that are suitable for both tall and short persons, one of the horizontal visible lines 32, 34 being used, as described heretofore by a tall golfer and the other line being used by a short golfer, to correctly align himself in a target direction and position the ball at the desired position between the feet.

Some glasses used strictly for putting a golf ball or a use requiring some other alignment that is only in the horizontal visual plane may have only one visible horizontal line on one or both eyepieces 18, 20, preferably displaced below the horizontal axis, as shown by the lines 22 and 36 in Figure 4.

There follows a brief description of embodiments of the invention for use in other sports. Basketball players during free throw frequently miss the basket by the ball being to one side thereof, i.e. either to left or right. This usually is caused by the hands being offset vertically on the ball at address and/or by the arms being incorrectly aligned with the basket during the throw, particularly during the last stages of the throw. Referring now to Figure 5, there is shown an embodiment of this invention having lines made like those of Figure 2 especially designed for use by the stationary basketball throwers, mainly during practice free throws, to easily check that alignment and to make adjustments or corrections, if necessary, during subsequent throws. In this embodiment, the right eyepiece 20 or the eyepiece for the dominant eye has a vertical visible line 38 thereon which passes through the visual axis of the eyepiece, while the other eyepiece has a horizontal visible line 40 thereon which passes through or near the visual axis of that eyepiece. The horizontal line 40 readily enables the player to check the horizontal alignment of the hands on the ball at address, i.e. just before the throw. If the hands are vertically offset, an adjustment can readily and quickly be made to bring the hands into proper horizontal alignment before the throw. During the last stages of the throw, the vertical line 38 is used to check whether the arms are correctly aligned with the basket and, if not, to align the arms correctly during subsequent throws. The eyeglasses of the embodiment of Figure 5 also may be used by runners to achieve the objectives described heretofore for running.

Bowlers frequently or usually miss strikes because the throwing or casting arm is not correctly aligned with the target at ball release or cast. Figure 6 shows an embodiment having lines made like those of Figure 2 especially designed to enable the bowler to easily check that alignment and to make corrections or adjustments, if necessary, to achieve correct alignment during subsequent throws or casts. In this embodiment there is a visible vertical line 38 on the vertical axis of the eyepiece for the dominant eye, e.g. the right eyepiece 20, while the other eyepiece 18 is devoid of lines. During the last stage of the throw or cast, the bowling arm of the bowler should be parallel to this line 38 which should be superimposed on the pins. If not, adjustments should be made in subsequent casts to achieve that result. The eyeglasses shown in Figure 6 also can be used by a tennis linesman to focus his attention on the line that he is responsible to judge.

Figure 7 shows an embodiment having lines made like those of Figure 2 especially designed for use by baseball batters. In this embodiment there are horizontal visible lines 40 on the eyepieces 18, 20 for both eyes. Both lines 40 pass through the visual axis of their respective eyepiece.

These eyeglasses aid the batter to achieve the objectives described heretofore for baseball batters.

A novel use of this invention lies in the embodiment shown in Figure 8 especially designed for use by a baseball umpire for pitchers to help visualize the strike zone. In this embodiment the right and left eyepieces 20 and 18 have vertical and horizontal lines 44 and 46, respectively, preferably clear or lightly tinted between two darker parts and both passing through the visual axis of their respective eyepiece. The lines 44 and 46 are relatively wide, however, so their apparent intersection superimposed on the view therethrough defines an area that approximates the size of the strike zone when the umpire is viewing the plate through that area from his normal position. This aids the umpire to correctly visualize the strike zone and make correct calls.

Referring now to Figure 9 of the drawings, there is shown an embodiment of this invention useful for a purpose different from sports activities, more especially for typists. Typists frequently copy with a typewriter or a word processor line-by-line from material that is in printed, typed or script form. The copier frequently glances from the material being copied to the typed material, i.e. on the paper in the typewriter or appearing on the screen or monitor of the word processor, to check copy ing accuracy. In so doing, the copier may lose the place in the material being copied. Figure 9 shows an embodiment of this invention to prevent such losses. This embodiment has a straight visible line 48 on one of the eyepieces extending from side to side and parallel to the horizontal axis thereof. The line 48 is on the left eyepiece 18 as shown, but it may be on the right eyepiece 20 depending on the relative positions of the material being copied and the typewriter or word processor screen. Preferably, the other eyepiece is clear and has no visible line thereon. The horizontal visible line 48 is defined by a clear section between two lightly tinted sections somewhat like the embodiment shown in Figure 1. In the embodiment of Figure 9, the width of the line 48 desirably is of the order of the height of the lines of the material being copied when the line 48 is superimposed thereon and may pass through the horizontal axis of the eyepiece or be displaced only slightly therebelow. In any event, the horizontal visible line 48 is preferably so located that when a typist is wearing the glasses and the line 48 is superimposed on the material line being copied, the typist, by lowering the head, can move the visible line 48 step-by-step from one line to the next of the material being copied and so facilitate accuracy of the copying task, i.e. avoid duplication or skipping a line. Further, without moving the head, the typist can glance at the copied material in the typewriter or on the screen to check copying accuracy and then move the eyes back to the same line on the copying material because the horizontal visible line 48 is superimposed thereon. Thus, loss of place is minimized.

It thus will be seen that the objects and advantages of this invention have been fully and effectively achieved. It will be realized, however, that the foregoing specific embodiments have been disclosed only for the purpose of illustrating the principles of this invention and are susceptible of modification without departing from such principles. Accordingly, the invention includes all embodiments encompassed within the spirit and scope of the following claims.

Claims

CLAIMS :

1. Eyeglasses having left and right transparent eyepieces, one of said eyepieces having thereon at least one transparent visible line of measurable width substantially parallel to one of the horizontal and vertical axes of said one eyepiece, said line appearing to be superimposed on any view through said one eyepiece and the area of said one eyepiece unoccupied by said at least one line being uncluttered by any obstruction to vision through said area.

2. The eyeglasses defined in claim 1 wherein the at least one visible line is parallel to the horizontal axis of the one eyepiece.

3. The eyeglasses defined in claim 2 wherein the at least one visible line is displaced below the horizontal axis of the one eyepiece.

4. The eyeglasses defined in claim 1 wherein the at least one visible line is substantially parallel to the vertical axis of the one eyepiece.

5. The eyeglasses defined in claim 4 wherein the at least one vertical line is displaced from the vertical axis to one of the nasal and temporal sides of the one eyepiece.

6. The eyeglasses defined in claim 2 including a visible transparent line of measurable width on the other of said eyepieces substantially parallel to the vertical axis of said other eyepiece.

7. The eyeglasses defined in claim 6 wherein the angle between the lines on the left and right eyepieces is about 90°.

8. The eyeglasses defined in claim 2 wherein the at least one visible line extends substantially from side to side of the one eyepiece.

9. The eyeglasses defined in claim 4 wherein the at least one visible line extends substantially from top to bottom of the one eyepiece.

10. The eyeglasses defined in claim 2 including another transparent visible line of measurable width on the one eyepiece spaced from and substantially parallel to the at least one visible line.

11. The eyeglasses defined in claim 1 wherein the at least one visible line is defined by at least one contrasting shade.

12. The eyeglasses defined in claim 1 wherein the at least one visible line is substantially straight.

13. The eyeglasses defined in claim 6 wherein the visible line on the other eyepiece is substantially straight.

14. The eyeglasses defined in claim 6 wherein the visible lines on the left and right eyepieces are substantially straight.

15. The eyeglasses defined in claim 2 wherein the at least one visible line is substantially continual.

16. The eyeglasses defined in claim 1 wherein the at least one visible line is on the left eyepiece.

17. The eyeglasses defined in claim 1 wherein the at least one visible line is on the right eyepiece.

18. The eyeglasses defined in claim 5 wherein the at least one visible line is displaced from the vertical axis toward the temporal side.

19. Eyeglasses especially designed for use by golfers, said eyeglasses having left and right eyepieces, the eyepiece for the dominant eye having thereon a transparent visible line of measurable width substantially parallel to the vertical axis of said eyepiece and displaced toward the nasal side thereof and the other eyepiece having thereon a transparent visible line of measurable width substantially parallel to the horizontal axis of said other eyepiece and displaced therebelow, said lines appearing to be superimposed on any view through the respective lines and the areas of said eyepieces unoccupied by said lines being uncluttered by any obstruction to vision through said areas.

20. Eyeglasses especially designed for use by basketball players mainly to practice free throws, said eyeglasses having left and right transparent eyepieces, the eyepiece for the dominant eye having thereon a transparent visible line of measurable width substantially parallel to and passing substantially through the vertical axis of said eyepiece and the other eyepiece having thereon a transparent visible line of measurable width substantially parallel to and passing substantially through the horizontal axis of said other eyepiece, said lines being superimposed on the views through the respective eyepieces and the areas of said eyepieces unoccupied by said lines being uncluttered by any obstruction to vision through said areas.