OF ARTS AND SCIENCES. 483 



Investigations on Light and Heat, made and pubushed wholly or in part with 

 Appropriation from the Rumford Fund. 



XXV. 



CONTRIBUTIONS FROM THE PHYSICAL DEPARTMENT OF THE 

 MASSACHUSETTS INSTITUTE OF TECHNOLOGY. 



XIX. — PRINCIPLES INVOLVED IN THE CONSTRUC- 

 TION OF PHOTOGRAPHIC EXPOSERS. 



By William H. Pickering. 



Communicated January 14, 1885. 



There are many forms of photographic exposers in use, in this 

 country and abroad, and they differ among themselves in many im- 

 portant particulars. The object of this paper is to determine from 

 theoretical considerations the general fundamental princijjles which 

 should govern their construction, and to suggest what seems to the 

 writer the best practical form of exposer where very rapid action is 

 desired. 



(L) Position of the Exposer. — The exposer may be placed either 

 just in front of the plate, or just back of the lens, or between the lenses, 

 or, finally, just in front of the lens. The former position is that gen- 

 erally employed by astronomers for taking photographs of the sun 

 itself It has the advantage that, by using a very narrow slit, the 

 observer may make the ex2:)0sure for any one point as short as he 

 pleases ; but different parts of the picture will be taken at different 

 instants, so that, if the body is in I'apid motion, the final result will be 

 distorted, and not represent the condition of things at any particular 

 instant. Moreover, this would be an inconvenient position for a 

 shutter in an ordinary camera, and the same remark applies to the 

 second place referred to, — just behind the lens. The usual place is 

 in front of the lens. This position has the advantage of convenience, 

 and in general involves less alteration of the lens-tube than if the 

 exposer is placed between the lenses. On the other hand, it has the 

 disadvantages that it exposes one portion of the plate slightly before 

 the other, and that the shutter has a considerably greater distance to 

 travel, so that, if very short exposures are required, this is a serious 



