if;9 



quently found to be inseii^iule, bat, on the other hand, sciiilet cloth 

 and various other dyed articles are highly sensitive. By means of r 

 linear spectrum the peculiar dispersion of a red light produced by 

 chlorophyll, or sorne of its modiiications, may be observed not only 

 in a solution, but in a green leaf, or on a washed paper, or in a sea- 

 weed. 



The highly sensitive papers obtained by washing paper with tinc- 

 ture of turmeric, or a solution of sulphate of quinine, or some other 

 highly sensitive medium, display their sensibility in a remarkable 

 manner when they are examined in a linear spectrum. In these 

 cases, however, the paper produces a very striking effect when 

 merely held so as to receive a pure spectrum formed in the usual 

 manner, that is, with a slit parallel to the edges of the prisms. Such 

 a paper may be used as a screen for showing the fixed lines belonging 

 to the invisible rays, though they are not thus shown quite so well 

 as by using a solution. The extraordinary prolongation of the spec- 

 trum seen when it is received on turmeric paper, has been already 

 observed by Sir John Herschel, by whom it w^as attributed to a pecu- 

 liarity in the reflecting power of that substance- Of course it now 

 appears that the true ex])lanation is very different. 



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 analysis 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 loicerecl 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 opacit}' of a solid or 

 fluid with respect to the invisible ravs more refran<2,-ible than the 



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