208 Obituary Notices of Fellows deceased. 



fluid, just as if it had been water, being merely reflected here and there 

 from motes. But from a point about halfway between the fixed lines G 

 and H to far beyond the extreme violet, the incident rays gave rise to u 

 light of a sky-blue colour, which emanated in all directions from the 

 portion of the fluid which was under the influence of the incident rays. 

 The anterior surface of the blue space coincided, of course, with the 

 inner surface of the vessel in which the fluid was contained. The 

 posterior surface marked the distance to which the incident rays were 

 able to penetrate before they were absorbed. This distance was at 

 first considerable, greater than the diameter of the vessel, but it 

 decreased with great rapidity as the refrangibility of the incident rays 

 increased, so that from a little beyond the extreme violet to the end, 

 the blue space was reduced to an excessively thin stratum adjacent to 

 the surface by which the incident rays entered. It appears, therefore, 

 that this fluid, which is so transparent with respect to nearly the 

 whole of the visible rays, is of an inky blackness with respect to the 

 invisible rays, more refrangible than the extreme violet. The fixed 

 lines belonging to the violet arid the invisible region beyond were 

 beautifully represented by dark planes interrupting the blue space. 

 When the eye was properly placed these planes were, of course, projected 

 into lines." 



At a time when photography was of much less convenient 

 application than at present even wet collodion was then a novelty 

 the method of investigating the ultra-violet region of the spectrum by 

 means of fluorescence was of great value. The obstacle presented by 

 the imperfect transparency of glass soon made itself apparent, and this 

 material was replaced by quartz in the lenses and prisms, and in the 

 mirror of the heliostat. When the electric arc was substituted for 

 sunlight a great extension of the spectrum in the direction of shorter 

 waves became manifest. 



Among the substances found " active " were the salts of uranium 

 an observation destined after nearly half a century to become in the 

 hands of Becquerel the starting point of a most interesting scientific 

 advance, of which we can hardly yet foresee the development. 



In a great variety of cases the refrangibility of the dispersed light 

 was found to be less than that of the incident. That light is always 

 degraded by fluorescence is sometimes referred to as Stokes' law. Its 

 universality has been called in question, and the doubt is perhaps still 

 unresolved. The point is of considerable interest in connection with 

 theories of radiation and the second law of Thermodynamics. 



Associated with fluorescence there is frequently seen a " false 

 dispersion," due to suspended particles, sometimes of extreme minute- 

 ness. When a horizontal beam of falsely dispersed light was viewed 

 from above in a vertical direction, and analysed, it was found to 



