653,342. Optical sound recording apparatus. RADIO CORPORATION OF AMERICA. April 28, 1948, No. 11658. Convention date, May 29, 1947. [Class 40 (ii)] In an apparatus for optically recording sound on film, monitoring means are provided in the modulating and projection unit, Fig. 1, for indicating on a screen 40, the adjustment of the modulation, condition of the light, and type of record being recorded (i.e. positive or negative). The elements of the optical system are shown diagrammatically in Fig. 8, in which a lightsource 5 having a filament 6 provides light for the recording and monitoring systems, the light gathered by the condenser lenses 8 being projected through a recording aperture 9 and monitoring apertures 10, 11 of an aperture plate 13. The emergent light from these apertures is projected to the mirror 15 of a galvanometer modulator through an intermediate lens 16. The light from recording aperture 9 passes through a condenser lens 18 and a plate 19 having a slit 20, emergent light from which is projected on to the film 25 by an objective lens unit 26. The light from the monitoring apertures 10, 11 is received upon a lens mirror 35 which is a plano-convex lens silvered on the plane side. The mirror 35 refracts and reflects the light back along a path parallel to the axis of the lens 18 to a plane mirror 36 and then vertically to an achromatic lens 37 where a stationary image of the galvanometer mirror 15 is produced by the lens mirror 35. The monitoring light passes through lens 37 to a plane mirror 38 and then to another plane mirror 39 which directs light on to the back of a translucent screen 40, to produce a magnified vertically-vibrating image 41 and illuminate one of two square openings 44, 45. The monitoring apertures 10, 11 are laterally offset from each other by a distance corresponding to half the width of each aperture. When the galvanometer is tilted so that light along the upper edge of the aperture 9 corresponds with the slit 20, a negative record will be obtained upon normal development of the film, and the light through the aperture 10 appears as an image 52, Fig. 9, on the screen 40. At the same time light from the aperture 11 illuminates the upper opening 62 which may be marked with a letter N. When a positive record is being made, the galvanometer is tilted so that light along the lower edge of the aperture 9 corresponds with the slit 20, and light through the aperture 11 appears as an image 53, Fig. 10, on the screen 40, while light from the aperture 10 illuminates the lower opening 63 which may be marked with a letter P. The dotted lines 55 in Fig. 9 show the relative position of the direct positive monitoring image in Fig. 10, while the dotted lines 56 in Fig. 10 show the relative position of the negative monitoring image in Fig. 9. The images 52, 53 vibrate in a vertical direction in accordance with the amplitude of the signal recorded, 100 per cent modulation being indicated by the upper lines 58. As the amplitude of the signal varies, the application of noise reduction bias is indicated by the position of the lower edge of the beam the amplitude during the application of 100 per cent bias being indicated by the spacing and position of lines 60. For monitoring the filament 6 of the lamp 5, light co-incident with the recording axis 29 is reflected upwardly by a prism 47 through a spherical lens 48 to a mirror 49 which projects the light through the aperture 137, Fig. 1, to the screen 40 as shown by the image 51, Fig. 8. In the unitary construction illustrated in perspective in Fig. 1, the galvanometer 82 is shown pivotally supported on bearings 84, 85 on a bracket 83 mounted a base whose position is adjustable by screws 86, 87, the tilt of the galvanometer being adjustable by means of a screw 88. The main structure 90 has a projecting portion 72 which carries a tubular element 71 in which the lenses 8, 16 and aperture plate 13 are mounted. The structure 90 also carries lens 18 and plate 19 in a mount 91, and the objective lens 26. In the overhanging part 121 are mounted the mirrors 38, 39. The mirror 36 and lens 37 are mounted on a support 110, and the lens-mirror 35 on a support 106.