70 PROCEEDINGS OF THE AMERICAN ACADEMY 



IV. 



ON PHOTOGRAPHS OF THE SOLAR SPECTRUM. 



By Robert Amory, M.D. 



Read, :vray 25, 1875. 



. The photographs I now present to the Academy were taken by 

 the action of sunlight, thrown by a collimating lens (21 inches focal 

 length) upon one of Mr. Rutherford's ruled speculum plates, contain- 

 ing 1 2,080 lines to the inch, and reflected therefrom upon an achro- 

 matic lens of five feet focus ; the image Avas then received upon a 

 sensitized collodion film. It will be observed that the solar lines are 

 more numerous than those shown by the photograph-map (herewith 

 also presented) of Professor J. W. Draper, and published in 1872, 

 which was taken from a ruled speculum plate containing only 6,480 

 lines to the inch. One of my photograph plates has been enlarged to 

 the same scale as that of Professor Draper's, and comprises solar 

 lines whose wave-lengths, as compared with Angstrom's map, are 

 between 4,590 and 3,700. I likewise present photographs of the 

 absorption bands of Ur&nium acetate, in which the three bands can 

 be distinctly seen, as well also as the solar lines of that portion of the 

 spectrum. 



In the ajiparatus used for these experiments, I would call the atten- 

 tion of the Academy to the following observed fact: that, if the slit of 

 the collimator is placed exactly at the principal focus of the collimat- 

 ing lens, the solar lines are in focus for all the different orders of the 

 spectrum upon the true are of a circle. If, however, the slit is outside 

 of the principal focus, these solar lines are in focus on a curve which 

 is }iot the arc of a circle, but whose radius gradually becomes 

 shorter in the higher orders. If, again, the slit is inside of the 

 principal focus, the radius of the curve gradually becomes longer in 

 the higher orders. The diagram I present may, perhaps, illustrate 

 this fact more clearly. Any one may verify it by direct experiment. 



Moreover, in using a dark chamber of large width between the sen- 

 sitized film and the lens used for projection of the . image from the 

 grating, and placing between this projecting lens and the sensitized 

 film a dia[)hragm which allows only that portion of the spectrum 

 actually photographed to pass through, I have had no trouble from 

 diffused light. In my earlier experiments, a dark tube of the same 

 calibre as the diameter of the projecting lens would always mar the 

 sensitized plate, on account of the diffused light, no matter how care- 

 fully the tube was fitted with diaphragms. 



