OP ARTS AND SCIENCES. 489 



two or three turns, unwind with a nearly constant velocity, so that, 

 if there is to be much variation in the exposures, (for example, a 

 ratio greater than 1 to 3 or 1 to 4,) we must resort to complicated 

 gearing. For those who are satisfied with these small ratios and com- 

 paratively long exposures, as those who are engaged in photographing 

 yachts exclusively, a coiled spring leaves little to be desired, as it is 

 compMCt and readily carried. On the other hand, if one wishes to vary 

 the exposure through a large range, such as 1 to 100, or to get an 

 exposure of less than 5^ of a second, the drop-shutter arrangement 

 offers peculiar advantages. 



Such a shutter has been constructed in which the drop is four inches 

 and the diameter of the aperture seven eighths of an inch ; but by 

 attaching two two-inch brass spiral springs beneath it, and doubling the 

 velocity by means of a pulley, the speed has been increased from ^^^ sec. 

 to TToij sec. The tension of the springs may be adjusted, and any inter- 

 mediate exposure given. A string which is attached to the top of the 

 shutter passes over a pulley, and has a twenty-gram weight fastened to 

 its other end. This exactly balances the shutter when the springs are 

 released, permitting it to remain motionless in any position. This is 

 desirable for focusing, and also for hand exposures. By thus counter- 

 balancing the weight of the shutter, removing the brass springs and 

 pulley, and attaching small weights in their place, the length of the 

 exposure may be increased from ^^ sec. to ^ sec. A shutter con- 

 structed on these principles has been in use by me now for some 

 months, and works admirably. The exposures under similar circum- 

 stances can always he relied on, and never vary among themselves 

 more than ten per cent. Its total weight does not exceed a pound, 

 and it can instantly be adjusted to give any exposure from ^^^ to | 

 second, or to give hand exposures. 



