ON OUR KNOWLEDGE OF SPECTRUM ANALYSIS. 



299 



less refrangible tlian the incideut rays. Prof. Stokes made drawings of 

 certain obscm-e bands which were seen in that part of the spectrum Ijing 

 beyond the violet, and these correspond with impressions taken on sen- 

 sitised paper and prepared photographic plates by M. Becquerel. He 

 found likewise that glass excited a considerable absorption of the more 

 refrangible rays, but that quartz was of all bodies the one which trans- 

 mitted them most easily. 



By direct vision, using lenses and prisms of quartz, Prof. Helmholtz 

 has been able to see the obscure rays, the existence of which was proved 

 by Prof. Stokes and M. Becquerel. A very pure spectrum was observed 

 in such a manner that a second slit shut off all the distinctly luminous 

 rays from the eye of the experimenter. By receiving the rays on a screen 

 saturated with quinine sulphate solution, they are rendered very plainly 

 visible.* M. Esselbach has modified the process and made further obser- 

 vations.- Dr. J, "W. Draper has also repeated the experiments of 

 Becquerel and reproduced the ultra-violet spectrum on photographic 

 plates.^ This work has been continued of late years by his son, Dr. 

 Henry Draper of New York. 



M. Mascart has examined the ultra-violet portion of the solar spectrum 

 by means of photography, and has given a drawing of a normal spectrum 

 extending beyond H.'* Making use of a Babinet's goniometer of very 

 perfect construction with a prism and lenses of quartz, he substituted a 

 photogi'aphic plate for the eye-piece. By the use of gratings traced on 

 glass by M. Nobert he has made measurements of the wave-length of some 

 of the ultra-violet solar rays as well as of the lines of cadmium, the spec- 

 trum of this ruetal being remarkable for the range beyond H, to which 

 its rays extend. 



There is a regular diminution in wave-length from the solar line H, 

 396'7, to 221' 7, the extreme cadmium line. It was remarked that the 

 shortest wave-length, 221' 7, together with the longest of the visible undu- 

 lations, A, 760, constitutes with the intermediate vibrations a scale extend- 

 ing nearly two octaves. In the accompanying table the wave-length of 

 ultra-violet cadmium lines as measured by M. Mascart are given. 



Wave-length of lines in that portion of the spectrum of cadmium more 

 refrangible that the solar line H. Determined by M. Mascart : — 



I 



Dr. Henry Draper was the first who obtained photographs of the sun's 

 spectrum completely in focus and on one plate from G (wave-length 

 4307) to above O (wave-length 3-440), the photograph in this case being 

 twelve inches in length. He succeeded on another occasion in photo- 

 graphing from near h (wave-length 5167) to T (wave-length 303-2) 



> Pogg. Annal. vol. sciv. a pj^^i Trans. 1859. 



' Ibid. vol. xcviii. * AnnaU's de VEcole JVarmale, 1864. 



* The wave-lengths calculated by MM. Mascart and Cornu are stated in milUonths 

 of a millimetre, -while those quoted in the foregoing report are given in ten- 

 millionths. 



