Photographing Ultra-Violet Spectra. 95 



Dr. Henry Draper was the first who obtained completely suc- 

 cessful photographs of the sun's spectrum. He succeeded on one 

 occasion in photographing on one plate the rays extending from 

 near h (wave-length 516-7) to T (wave-length 303-2). The 

 spectrum was produced by means of a very perfect ruled glass 

 grating made by Mr. M. L. Rutherford of New York. The jaws 

 of the spectroscope slit were made of steel and were moveable 

 by a micrometer screw, the width of the slit used was jjrsth. of 

 an inch. 



In order to obtain a spectrum of uniform character for rays of 

 all refrangibilities, parts of the sensitised plate were protected by 

 a series of diaphragms, during exposure for faint groups of rays ; 

 by the removal of these at intervals the strong rays were photo- 

 graphed with a distinctness which could not otherwise have been 

 obtained. The sensitive plates used were prepared with bromo- 

 iodized collodion and sensitised with a nitrate of silver bath. 



Dr. Draper's fine photographs show how impossible it is to 

 depict the relative intensities of lines in the spectrum by any 

 other means than photography, and how groups of lines even 

 may fail to be resolved. In fact as he states " The exact cortipo- 

 sition of even a part of the spectrum of a metal ivill not he known 

 until lue have obtained pltotographs of it on a large scale!' 



Dr. Draper comments on the unsatisfactory nature of Miller's 

 photographs in the following words : — 



" I have also tried to utilize the photographic spectra of the late W. 

 A. Miller, published in the Philosophical Transactions for 1862, but for 

 some reason, probably insufficient intensity of the condensed induction 

 spark, his pictures do not bring out the peculiaiities of the various 

 metals in the striking manner that is both necessary and attainable." — 

 (" On Diffraction Spectrum Photography and the Determination of the 

 Wave-lengths of the Ultra-Yiolet Kays."— ^' Nature," 1874, Vol. xi., 

 p. 224. 



M. Cornu has given a description of the solar spectrum from 

 the line called h to the ray 0, and has drawn a beautiful map 

 made to the scale of wave-lengths. — (Annales de I'Ecole Normale, 

 1874). The spectra were observed by photography in a manner 

 similar to that devised by M. Mascart, but as the optical appara- 

 tus was made of glass all rays more refrangible than (wave- 

 length = 344-11) were intercepted. In continuation of his experi- 

 ments (Annales de I'EcoleNormale, 1880,) using more perfect lenses 

 of quartz and Iceland spar as well as prisms of these materials, 

 SciEN. Pkoc, Il.D,S. Vol. hi,, Pt. hi. I 2 



