686 REPORT — 1903. 



that they exhibited light and darkness. The screen used was of plaster of Paris 

 saturated with a Huorescent substance, such as uranium phosphate. 



William Allen Miller in 1863, simultaneously with Stokes, described his method 

 of examining the photographic transparency of various saline solutions and organic 

 substances and of depicting metallic spectra. A sensitised photographic plate 

 was used for the reception of the rays of the spectrum, so that they were made to 

 register their own position and intensity. L. Soret invented the fluorescent 

 eyepiece for the purpose of investigating the ultra-violet rays and ascertained the 

 best media for the transmission of rays of high refrangibility. Colourless fluor- 

 spar, a rare mineral, was found to answer best, and quartz lenses were achromatiaed 

 with this. Iceland spar was found to absorb some of the more refrangible rays, 

 and a pure spectrum was difficult to obtain with quartz prisms owing to double 

 refraction, which caused the lines in metallic spectra to be duplicated. Struck by 

 the fact that Miller had examined many organic substances without obtaining 

 evidence of a connection between their constitution and their absorption spectra — 

 the actual words used by Miller were, ' I have not been able to trace any special 

 connection between the chemical complexity of a substance and its diactinic 

 power ' ' — it appeared to me desirable that this point should be systematically 

 reinvestigated. L. Soret had already proceeded with work in this direction, 

 by examining and drawing a great variety of organic substances and diagrams of 

 absorption curves. But it was deemed necessary to make a large number of exa- 

 minations of substances of a comparatively simple constitution, and according to 

 theory closely related, and afterwards gradually to proceed to the study of substances 

 of greater complexity. For such purposes a photographic method alone appeared 

 a practicable one, particularly when comparisons had to be made between sub- 

 stances observed at different times, for the reason that none but photographic 

 records could be absolutely relied upon and stored away for future reference.- 



The plan of the proposed investigation was to photograph the rays transmitted 

 by molecular proportions of hydrocarbons, alcohols, acids, and esters, either alone 

 as vapour or liquid, or dissolved in some neutral and, in comparison with the sub- 

 stances to be examined, an optically non-absorbent solvent. 



' Journ. Chem. Soc. vol. xi. p. 68. 



■•^ Clerk Maxwell had calculated for Miller the best focal length of lenses of 

 quartz which would give an approximately flat field. His computation made this 

 something over a length of three feet. All Miller's photographs were taken with the 

 plate placed normal to the axis of the lens, but Stokes had shown that the locus of 

 the foci of the different rays formed an arc of a curve or nearly a straight line, 

 lying very obliquely to the axes of the pencils coming through the lens. 



It was obvious from Miller's photographs that only one or two rays on each plate 

 were even approximately in focus. To obtain spectra in focus from end to end it 

 was evidently necessary to incline the plate so that the end upon which the red rays 

 would fall, which are of longest wave-length, should be farther off than that upon 

 which the ultra-violet fall which are of shortest wave length. It was also found 

 experimentally that lenses of much shorter focal length (ten or twelve inches) could 

 be used, giving perfect definition, and, what is still more important, it was found a 

 positive advantage not to have them corrected by fluor-spar or calcite. The plate 

 carrier was adjusted at an inclination of approximately 22° to the normal ; in such a 

 position the rays from the yellow sodium line to the extreme ultra-violet of the 

 spark spectrum of cadmium were simultaneously in focus on a plane surface. 



The prism was of quartz cut on C.rnu's plan, the method of construction designed 

 to get rid of all double refraction being communicated to me by M. Cornu in a very 

 kindly written letter. 'I'he first instrument was constructed in 1878 and the 

 description of it published in 1881. It has been the model for several others. One 

 with two prisms and lenses of 12 inches focus was exhibited by me in the Inventions 

 ^Exhibition in 1882. At the Jubilee meeting of the British Association at York the 

 spark spectra of iron, cobalt, and nickel, enlarged to twenty-five diameters and 

 printed by the Autotype Company, were exhibited. They are over eight feet in 

 length, and have proved very useful for reference. The photographic process is a 

 point of great importance ; the then newly invented gelatine bromide films made by 

 Kennet wete alone quite suitable. 



