INSTANTANEOUS PHOTOGRAPHY. 13 



For recording the times and durations of exposure a chrono- 

 grapli was used, which had been designed by the writer for 

 the University of Pennsylvania. An electric current vibrating a 

 tuning-fork in the usual manner, an electro-magnet with a style 

 to its armature in the same current, on the smoked paper on the 

 turning cylinder a series of dots one one-hundredth of a second 

 apart, furnished the regular time-comparison intervals. Along- 

 side of this electro-magnet a similar one recorded the exposures. 



The horizontal cylinder on which the paper was stretched and 

 smoked could be turned by hand or clock-work, and the two 

 magnets were on a carriage, which travelled slowly from left to 

 right by an endless screw, as in the slide-rest of an engine lathe. 



The means of establishing the circuit which marks the openings 

 or exposures was as follows : An interrupter disk of brass and 

 hard rubber was fixed on the axis of the disks, and is turned on 

 the axle until the brass may correspond to the opening of the 

 disks and there clamped. 



A spring with platinum contact point, and capable of move- 

 ment in two slots at right angles, is also adjusted according to the 

 number of openings and sectors of brass. 



The circuit carried through the interrupter is taken to the key- 

 board that controls the shutter, and the pressing the first key 

 that breaks the contact holding the first slide allows contact in 

 the exposure-marking circuit. The key carries on a spring a 

 platinum point pressing against either of two metal strips accord- 

 ing to its position. 



The inner straight edges of these metal strips are not parallel, 

 but approach at one end ; and the contact spring being fastened 

 to the key-bar by screws through a slot instead of holes, the 

 spring may be pushed back and forth along the bar, and so ad- 

 justed that thei'e is no appreciable interval between leaving the 

 first contact and gaining the second. 



Thus the counting begins as soon after the release of the first 

 shutter-slide as an opening comes opposite the lens. These keys 

 have also the ordinary adjusting-screws of the telegraphic key. 



The pressing of the second key, which releases the second 

 shutter-slide and stops the lens, breaks the exposure-counting 

 circuit by means of a see-saw between the two keys, which throw^s 

 the first key back into its first position. 



