Dr. H. Draper on Diffraction- Spectrum Photography. 425 



fine lines is seen. In the group L he shows 12 lines, while 

 even in the Alberttype copy of my photograph 25 can be counted, 

 and in the original negative many more. From H to L he shows 

 70 lines; in my Plate 138 can be observed besides many unre- 

 solved bands. 



In the earlier part of this memoir it was stated that the rela- 

 tive intensities of the lines in the spectrum were correctly repre- 

 sented if a certain allowance was made. If an unshielded col- 

 lodion plate were presented to the image of the spectrum, there 

 would be produced a stain very dense from G to H, fainter above 

 H, and still fainter below G. But this stain would not represent 

 the actinic force of the sun, it would merely be an index of the 

 decomposability of a mixture of iodide and bromide of silver. I 

 have for this reason adopted the idea of J. W. Draper, that force 

 is equally distributed through the spectrum, and have tried to 

 produce a photograph of equal intensity throughout. This has 

 been accomplished, as I have before stated, by suitable dia- 

 phragms. But whether this view be correct or not, lines which 

 are not far distant from one another are presented virtually with- 

 out any interference by diaphragms, and must therefore be cor- 

 rect both as to shading and intensity. 



Besides the points above mentioned, there are many theore- 

 tical considerations suggested by the photograph which it does 

 not seem expedient to enter upon fully at present. Among such 

 is the possibility of arriving at an estimate of the sun's tempe- 

 rature by interpreting the apparent bands, such as those near 

 G and H, by the aid of Lockyer's researches on the temperature 

 of dissociation of compounds. No one has yet ascertained 

 whether there are or are not unresolvable bands in the solar 

 spectrum. If they do exist, the compounds to which they be- 

 long, and the necessary temperature for dissociation, remain to 

 be determined. 



It would seem also to be possible to find out whether, as as- 

 serted by Zollner, there is a liquid envelope round the sun, by a 

 search for more diffused bands in its photographed spectrum. 



In the hope that this photograph may prove to be of value to 

 scientific men for further investigations upon the sun and the 

 elements, I have caused a number of extra copies to be printed, 

 and shall be glad to present them to any one who can make use 

 of them. 



University, Washington Square, 

 New York. 



