355,418. Tell-tale cameras. THRING, L. G. P., 8, Park Terrace, Cambridge. June 19, 1930, No. 18739. [Class 98 (i).] A photographic recording apparatus for use with a measuring instrument comprises a recording camera having a drum rotatably mounted on bearings and with a film or other light sensitive-surface wrapped round it, a spindle forming a connection between the drum and the operating mechanism for rotating the drum, and a light-proof casing within which the drum is removably mounted, the drum and casing forming a removable unit. The light-proof casing in held stationary when the drum is in its operative position in the camera and is provided with a slot controlled by a shutter through which a beam of light can pass to the film. A base member within the camera is provided to hold the casing, and means are provided for preventing the casing being placed in position or removed from the base except when the shutter controlling the slot is in closed position. The record strip may be graduated by a spark arrangement in association with an optical system which projects an image of the spark on to an edge of the record strip. The apparatus as shown in Figs. 1 and 2 comprises a body A provided at its front end with an objective A<1> and carried at its rear by a plate B supported on a base C. The base C has mounted thereon operating mechanism for rotating a recording drum D and a source of light E adapted to project a beam of light through a slot in the plate B on to a mirror on the instrument under observation and back through a lens B3 and slot to the recording drum. The base C carries two bearing plates J, J<1> spaced apart and having bearings H<1>, H<2> for a driving shaft H. The recording drum D is rotatably mounted on a hollow shaft M surrounding the shaft H and is adapted to be secured to rotate with the shaft H by means of a slotted cross-bar D<4> screwed to the drum and engaging the flattened part H<4> of the shaft. The drum is enclosed in a light-proof casing comprising two cylindrical members K, O closed by end plates L, L<1>. The upper end plate has a flange L<3> cooperating with a flange D<5> on the cross-bar to exclude the light when the drum unit is withdrawn from the shaft. The lower end plate is provided with a boss L<2> adapted to fit into a boss J<3> on the upper bearing plate J and can be located correctly thereon by means of an inter-engaging slot L' and pin J<4>. The spindle H extends beyond the upper end plate and is provided with means for attachment to a tachometer N. The members K, O are mounted for limited rotational movement to align slots K<2>, O<1>, and the lower end plate L<1> is provided with three radial pins L<5> adapted to engage corresponding semicircular recesses O<2>, Fig. 4, in the member O. An internal rim K<3> on the member K which supports the member O is provided with corresponding bayonet slots K<4> through which the pins L<5 >can pass when the parts are fitted in position. The outer member K is locked to the lower end plate L<1> when the member K is rotated. The member K is provided with radial pins K<5>, Figs. 3 and 4, adapted to co-operate with a slotted ring R, Figs. 3 and 4, secured to the bearing plate J. The arrangement of pins and slots is such that the pins K<5> and slots R<1> are only in alignment when the outer cylinder K is in the position in which the slot K<2> is out of register with the slot O<1> of the inner cylinder O and the bayonet joint which connects the two cylinders to the endplate L<1> is in its locked position. When the outer cylinder K is turned to lock it in position the slots K2, O are brought into register. When the film is to be removed from the drum the unit is placed upon a plate with pins similar to J3, J<4> to hold it while the casing is turned to unlock the bayonet joint. A tubular passage S mounted in the casing A contains one or more spark gaps T, Fig. 6, associated with an optical system of lenses, mirrors and prisms for transmitting the image of a spark and focusing it on to the edge of the film. A second optical system may be utilized to project an image of a spark through an aperture Y for observation purposes. The sparks may be produced by any suitable circuit arrangement such as a magneto, discharge of condensers, or other means, the circuits being connected to a tuning-fork or chronograph to regulate the time. The shutter El through which the beam of light passes may also be controlled by an operating mechanism the timing of which may have a desired relationship with the occurrence of a spark.