731,943. Spectroscopic apparatus. LEEDS & NORTHRUP CO. July 30, 1952 [Aug. 10, 1951; Aug. 10, 1951; Aug. 10, 1951], No. 19281/52. Class 97 (1). [Also in Groups XXXVII, XL (a) and XL (b)] An optical system, particularly for spectroscopic apparatus, comprises a concave spherical surface having two reflecting portions, one receiving radiant energy from an object and reflecting it to a reflector facing the said surface and the other portion receiving the radiant energy reflected by the reflector and directing it to form an image of the object at a distance from the concave spherical surface substantially equal to one half its radius of curvature, image and object being disposed equidistant from the axis of the spherical surface on opposite sides thereof, and radiant-energy responsive means for producing an electric signal disposed to receive light from the said image. Figs. 1 and 3 show the system applied to a spectroscope having means for scanning the spectrum across the exit slit. Light from an arc or spark 10 passes the entrance slit 11, Fig. 1, is collimated by the portion 12a of a concave spherical mirror 12 and falls on a reflecting grating 13. The dispersed beam is reflected to the portion 12b of the mirror 12 which forms an image at the exit slit 14. The intensity of a spectral line passing the slit 14 may be measured by means of a photo-multiplier tube 15 and recorder 18 and may be compared with the intensity of the beam from the slit 11 by the aid of a second photo-multiplier tube 16 receiving light by reflection in a semi-transparent mirror 17. In Fig. 3 the entrance and exit slits 46, 47 are both curved to circular arcs to minimize the effects of astigmatism produced by the concave mirror 12. Fig. 8 shows an arrangement whereby the curved slits may be variable. A base plate 49 is provided with apertures 50, 51 covered in part by a plate 53 having a circular aperture 54, and in part by members 56, 57 slidable simultaneously or separately in guides 58, 59 by screws 62, 63, and having curved edges parallel to those of the aperture 54. The spectrum is scanned across the exit slit 14, Fig. 1, continuously or in steps by rotating by means of an arm 41, a shaft 40 carrying the reflection grating. The stepwise movement is produced by a motor 80, which rotates a turret 68 so as to allow one of a number of rods 67, of different length, to engage the end of the arm 41, the latter being moved away from the rods 67, when the turret is to be rotated, by a cam 69, against a spring 74, driven by a motor 72. Also driven by the motor 72 is a cam 101 operating an arm 102 on a member 104 bearing the entrance slit 11 so as to rotate the latter slightly during each dwell of the arm 41 on a rod 67 to allow each spectrum line to be moved across the exit slit to permit determination of its maximum intensity. Fig. 3 shows the arrangement for continuous scanning of the spectrum which is effected by a member 125 engaging the arm 41 at one end and being in screw-threaded engagement at the other end with a drive member 123 rotated by a shaft driven from a motor 81 through the gearing shown. This comprises three gears 83, 84, 85 engaging respectively gears 87, 88 and 89, which may be selectively clutched to the shaft 86 to allow the member 125 to rotate at one of three different speeds. An arm 128 on the member 125 operates switches 129 and 130 to stop the motor 81 when the arm 41 reaches one or other of the limits of its motion, and a reversing switch 118 allows the arm 41 to be moved in either direction. The apparatus may be provided with both continuous and stepwise movement mechanisms for alternative use, and an electrical control system which permits automatic operation of these movements through a single sequence of spectral lines or through repeated sequences is shown in Fig. 18. The turret 68 is rotated by an electric motor 271a driving a shaft 294 operating a cam 69 to move the arm 41 clear of the pins 67 whilst an electric motor 80 operates a shaft 336 to move the turret from one position to the next, the movements of the two motors being governed automatically by the switches and circuit shown. A switching arrangement is provided to allow operation of the spectroscope to commence with a certain pin 67 in use, to continue automatically until a subsequent selected pin is reached and then either to cease automatically, or to repeat the cycle once more automatically. Reaching starting position. To set the circuit to commence operation with a certain pin 67 in use a switch 400 is rotated by a knob 338 until the contact arm thereof is opposite the appropriate position. In this position all contact points except the selected one of the switch 400 are in circuit with the corresponding points of a switch 401 the contact arm of which is driven by the shaft 426 of the rotating turret 68. With the switches in the position shown, if power is connected to the leads 301 and the contact 341 is not in the selected position for starting the cycle, the motor 80 is energized through contacts 227b, switches 400 and 401, leads 418 and 413, contacts 273c and 402b and a lead 414 to rotate the turret, and thence the shaft 426, until the contact arm reaches the appropriate contact. This breaks the circuit through switch 340 but supply to the motor is maintained temporarily by the closure of a contact 404b of a microswitch 404a on the shaft 426. When the pin 67 is accurately located the disc 404a breaks the contact 404b, thereby cutting off the motor 80, and closes a contact 404c which connects the motor 80 to a condenser 407 which has been charged through rectifier 409 and resistance 401 from the supply 301. The condenser discharges through the motor 80 and brings it to an abrupt stop. Operating cycle. To commence a cycle of operations, a contact 232 is closed energizing a relay 230 the contact 229 of which closes a holding circuit through switches 410, 411, a second holding circuit through switch 412, opens contact 227b and closes contacts 227a and 226a, the latter closure causing the motor 271a to rotate and to begin to operate the cam 69. After the latter has rotated to allow the arm 41 to dwell on the first pin 67 long enough for one spectral line to be measured a cam 273a on shaft 294 of motor 271a momentarily closes a switch 273b to complete the circuit to motor 80. The latter performs a small rotation sufficient to cause the microswitch to close the contact 404b and maintain the motor in operation until the next pin is in position when closing of the contact 404c and discharge of the condenser 407 stops the motor. Continued rotation of the motor 271a repeats the cycle of operations until the contact arm 345 of switch 411 engages the contact corresponding to the setting of switch 410, similar to switch 400, which has been adjusted to the last pin'to be used. This breaks one of the holding circuits to the relay 230. The other circuit, through the switch 412b is broken by the cam 412a when the motor 271a has moved the arm 41 clear of the pins 67. Breaking of the holding circuits opens the contact 226a to the motor 271a and connects it to a switch 277e operated by a cam 277c to connect the motor to a charged condenser 258, discharge of which brings the motor 271a to an abrupt stop. Release of the relay 230 also closes contact 227b completing the circuit to motor 80 through switches 400 and 401 so that, as previously described the motor 80 rotates the turret 68 until the first selected pin is once more in position and the apparatus ready for another operating cycle. To provide for the automatic repetition of the cycle a third holding circuit is employed comprises corresponding switches 415, 416, the former set in position by the knob 338 and the latter moved by the shaft 426. When the motor 80 has returned the turret 68 to its starting position switches 416, and 415 are connected together to maintain the holding circuit of the relay 230. The shaft 426 also operates a contact 265 connected to electrical elements in the recorder to which photo-multiplier 15 is connected, the contact 265 automatically adjusting the response &c. of the recorder according to the wavelength under examination. To avoid errors due to the wandering of the 'spark over the surface of the electrodes when spark illumination is employed slits 529, 537, Fig. 21, are located between the source and the reference photo-multiplier tube 16, the slit 537 being adjustable in width and the spacing of the elements being suitably driven. Alternatively the variable slit 537 may be omitted. Specification 732,058 is referred to.