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



the lines of the spectrum from near G to 0, or from wave-length 

 4350 ten-millionths of a millimetre to 3440. Above that is 

 placed a scale which is a copy of Angstrom's, from just below G 

 to H 2 , with the same-sized divisions carried out from H 2 to 0. 

 The second, or lower, is a magnified portion of the same negative, 

 having H T and H 2 about its middle, and extending from wave- 

 length 4205 to 3736. 



It follows, therefore, that' the lines in the solar spectrum are 

 correctly represented in their relative positions. The only errors 

 are those which may have arisen from mal-adjustment of the 

 scale. The precautions that were taken to avoid such errors 

 will be described. With a certain correction, to be mentioned 

 hereafter, it may also be stated that the relative shadings and 

 intensities are preserved. 



The value of such a map depends on the fact that it not only 

 represents parts of the spectrum which are with difficulty per- 

 ceived by the eye ^though they may be seen by the methods of 

 Stokes and Sekulic), but also that even in the visible parts there 

 is obtained a far more correct delineation in those portions w T hich 

 can be photographed. In the finest maps drawn by hand, such 

 as those in the celebrated " Spectre Normal du Soleil" of Ang- 

 strom, the relative intensity and shading of the lines can be but 

 partially represented by the artist, and a most laborious and 

 painstaking series of observations and calculations on the part of 

 the physicist is necessary to secure approximately correct posi- 

 tions of the multitudes of Fraunhofer lines. Between wave- 

 lengths 3925 and 4205 Angstrom shows 118 lines, while my 

 original negative has at least 293. 



For such reasons many attempts have been made to procure 

 good photographs of the diffraction-spectrum. The earliest were 

 by my father, J.W. Draper ; his results were printed in 1843-44 

 in a work entitled " On the Forces which produce the Orga- 

 nization of Plants." This memoir was accompanied by plates 

 drawn from his daguerreotypes ; and the wave-lengths, which he 

 first suggested as the proper indices for designating the Fraun- 

 hofer lines, were used as a scale. 



Since that time the most important experiments in this direc- 

 tion have been by Mascart and Cornu. These eminent physi- 

 cists, however, have resorted to the plan of taking portions of the 

 spectrum on a small scale, and subsequently making enlarged 

 drawings therefrom. This course introduces the defects of hand 

 work, and the artistic difficulties of copying intensity and shading, 

 as well as the omission of fine lines. 



In the photographs of the spectrum which I have taken, I 

 have tried to get as large a portion as I could at once, and on 

 as large a scale as possible, I have usually obtained images 



