704 Mr. James J. Dobbie [April 4, 



the great difficulties that must have been encountered in observing 

 and measuring accurately the position of bands on a luminous screen. 



While Stokes was engaged in these researches, Prof. William Allen 

 Miller was simultaneously at work in the same field, and Stokes left 

 the further development of the subject in his hands. Instead of a 

 fluorescent screen, Miller employed a sensitive photographic plate for 

 the reception of the spectrum. The rays of light were by this means 

 made to register their own position and intensity, and a permanent 

 record was obtained which could be studied at leisure. Miller failed, 

 however, to " trace any special connexion between the chemical com- 

 plexity of a substance and its diactinic power." Struck by this fact, 

 W. N. Hartley — now Sir Walter Hartley — commenced a systematic 

 investigation of the whole subject, and it is to his researches, extend- 

 ing over a period of more than thirty years, that we owe, not only 

 most of the knowledge which we now possess of the relation between 

 the structure of organic substances and the action of such substances 

 on the ultra-violet rays, but the elaboration of the convenient and 

 elegant methods by which such investigations are now conducted. 



The light derived from an ordinary source of illumination, such 

 as an electric lamp, consists of waves of all degrees of refrangibility, 

 and its spectrum shows a continuous band of colour ranging from 

 red to violet. The limits of this visible spectrum lie between the 

 wave-lengths 7600 and 8900. 



If, instead of the electric light or other ordinary source of illu- 

 mination, we employ the light emitted by one of the metals when 

 raised to a high temperature, the spectrum is seen to consist of a 

 series of lines of different colours and intensities lying within the 

 same limits as the visible spectrum. But there are rays Ijeyond the 

 red end of the spectrum and rays beyond the violet end which excite 

 no sensation of luminosity in the eye. By allowing the spectrum 

 to fall upon a screen which has been coated with a fluorescent sub- 

 stance, such as sulphate of quinine or a salt of uranium, these rays 

 are rendered visible for a short distance beyond the violet. But it 

 is only when we replace the glass apparatus, with which we have 

 hitherto been working, by a quartz prism and lenses, and substitute 

 a photographic plate for the eye, that the fall extent of the spectrum 

 beyond the violet is revealed (Fig. 1). This is the ultra-violet region 

 — the region which Stokes opened up to investigation, and it is with 

 the behaviour of organic substances towards the rays of this part of 

 the spectrum that we have mainly to do this evening. 



When light is transmitted through a coloured solution certain 

 rays are absorbed, and dark l)ands corresponding to these rays appear 

 in the spectrum The importance of these bands as a means '^ 

 distinguishing coloured substances has long been recognized, and, : 

 we have already seen, considerable progress had been made with 

 their study fifty years ago. As the bands in this case are in the 

 visible spectrum, no special means are required for their observation. 



