280 PROCEEDINGS OF THE AMERICAN ACADEMY 



quite easily made out, especially if viewed through a magnifying lens of 

 low power. It may also be noticed that the two lines of D are quite 

 sharp, and distinctly visible : possibly the positive impression taken 

 from the same negative, which is here exhibited, may make these lines 

 more visible in the dazzling gaslight of this room. I also present 

 negatives and their positives of the absorption bands of a solution of 

 uranium acetate, of" uranium chloride, and also of a solution of chloro- 

 phyll, the latter of two kinds, one of which is obtained from grass, 

 and the other from green tops of the asparagus-plant. 



In conclusion, I will state that we have used the most simple appara- 

 tus that could be devised, so that we may only lose a small portion of 

 light, and not complicate the results by too many reflections of the 

 image submitted to the photographic action. This apparatus consists, 

 1 st, of the heliostat ; 2d, a mirror arranged in the central axis of the 

 heliostat to intercept and reflect the beam into a dark room ; 3d, a 

 slit (width about one-hundredth of an inch) in the shutter of the dark 

 room ; 4th, a collimating lens at its exact focal distance from the slit ; 

 5th, a dense glass jirism arranged at the minimum angle of deviation 

 for the especial line whose image we wish to photograph ; Gth, a com- 

 mon spectacle lens of forty-two-inches focus ; 7th, the sensitized plate. 

 Between the prism and the sensitized plate, a square, dark box of 

 large capacity excludes all other light than that we wish to use. 



On the following page will be seen heliotype prints of these bands, 

 though it should be borne in mind that it is almost impossible to ob- 

 tain an exact reproduction of details in a positive on paper, unless the 

 negative be retouched, which for obvious reasons cannot be allowed. 

 Mr. E. PLdwards very obligingly has devoted much pains to the repro- 

 duction of these jirints by the heliotype process. 



