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1908-9. | A Special Form of Photographic Camera. 
the axle O. On completing a revolution the projecting end of P comes in 
contact with a fixed stop R, and is depressed, which raises the other end 
free from Q, thus releasing the small wheel from the axle, so that the 
carriage can run back to its starting-point, the small wheel turning back- 
wards freely on the axle until its ratchet once more on the completion of 
a revolution re-engages with Q. 
The return of the carriage C (fig. 1) brings about also the vertical move- 
ment of the plate. It will be noticed that the frame F containing the dark 
slide is furnished with a row of 6 catches T, T, . . . , each of which in turn 
engages with a stop V fixed to the carriage C. When C runs back along 
DE, the stop W fixed to A strikes against the catch which is in action for 
the time being, and pushes it clear of V, thus permitting of the descent of 
the frame F until the catch next above in its turn engages with Y. 
Fig. 4 shows a full-size reproduction of a negative, from which it will 
be seen that the record made by the camera is a very legible one. 
Fig. 3. — M is the cord to the carriage, which is wound round the grooved 
circumference of the wheel S. 
In order that the observer may be able to identify individual photo- 
graphs, or to indicate the commencement of a new series of records, a means 
is provided whereby he can produce at will on the photographs either a 
single or a double wedge-shaped mark. Two small levers are pivoted so that 
either may be swung in front of the scale into the field of view of the 
camera by a touch of the observer’s finger, their ends being respectively 
in the shape of a single and of a double wedge. In my own work I do 
not, as a rule, require to use these marks, as 1 always take the average 
of ten readings, and the record of each experiment occupies just one line 
on the plate. In order to relieve the observer from having to count 
the exposures, I have arranged an electric bell which rings at each return 
of the carriage when the ten exposures have been made. The observer is 
thus able to give his undivided attention to the work of adjusting the 
polarimeter, and the speed at which the readings can be made is greatly 
