392 Royal Society. 



A high degree of sensibility appears to be rather rare among inor- 

 ganic compounds. Certain specimens of fluor spar, as is already 

 known, give a copious internal dispersion of a deep blue light ; but 

 this is plainly due to some foreign ingredient, the nature of which is 

 at present unknown. But there is one class of inorganic compounds 

 which are very remarkable for their sensibility, namely, certain com- 

 pounds of peroxide of uranium, including the ornamental glass called 

 canary glass, and the natural mineral yellow uranite. In these com- 

 pounds the dispersed light is found on aiaalysis to consist of bright 

 bands arranged at regular intervals. A very remarkable system of 

 absorption bands is also found among these compounds, which is 

 plainly connected with the system of bright bands seen in the spec- 

 trum of the dispersed light. The connection between the absorption 

 and internal dispersion exhibited by these compounds is very sin- 

 gular, and is of a totally different nature from the connection which 

 has been already mentioned as occurring in solutions of the green 

 colouring matter of leaves. 



There is one law relating to the change of refrangibility which 

 appears to be quite universal, namely, that the refrangibility of light 

 is always loivered by internal dispersion. The incident rays being 

 homogeneous, the dispersed light is found to be more or less com- 

 posite. Its colour depends simply on its refrangibility, having no 

 relation to the colour of the incident light, or to the circumstance that 

 the incident rays were visible or invisible. The dispersed light appears 

 to emanate in all directions, as if the solid or fluid were self-luminous 

 while under the influence of the incident rays. 



The phenomenon of the change of refrangibility of light admits of 

 several important applications. In the first place it enables us to 

 determine instantaneously the transparency or opacity of a solid or 

 fluid with respect to the invisil)le rays more refrangible than the 

 violet, and that, not only for these rays as a whole, but for the rays 

 of each refrangibility in particular. For this purpose it is sufficient 

 to form a pure spectrum with sun-light as usual, employing instead 

 of a screen a vessel containing a decoction of the bark of the horse- 

 chestnut, or a slab of canary glass, or some other highly sensitive 

 medium, and then to interpose the medium to be examined, which, 

 if fluid, would have to be contained in a vessel with parallel sides 

 of glass. Glass itself ceases to be transparent about the region 

 corresponding to the end of the author's map, and to carry on these 

 experiments with respect to invisible rays of still higher refrangibility 

 would require the substitution of quartz for glass. The reflecting 

 power of a surface with respect to the invisible rays may be examined 

 in a similar manner. 



The effect produced on sensitive media leads to interesting informa- 

 tion respecting the nature of various flames. Thus, for example, it ap- 

 pears that the feeble flame of alcohol is extremely brilliant with regard 

 to invisible rays of very high refrangibility. The flame of hydrogen 

 appears to abound in invisible rays of still higher refrangibility. 



By means of the phenomena relating to the change of refrangibi- 

 lity, the independent existence of one or more sensitive substances 

 may frequently be observed in a mixture of various compounds. In 



