PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 537 
Application of internal dispersion to demonstrating the course of rays. 
193. Solutions of quinine have already been employed for this purpose, and a weak 
decoction of the bark of the horse-chestnut appears to be decidedly better. But the 
effect is immensely improved by using absorbing media to cut off all the rays belong- 
ing to the bright part of the visible spectrum. A deep blue glass will answer very 
well for this purpose if its faces be even, so as not to disturb the regularity of the 
refraction. The appearance of the general pencil refracted through a rather large 
lens, vrith its caustic surface, its geometrical focus, &c., is singularly beautiful when 
exhibited in this way, on account of the perfect continuity of the light, and the deli- 
cacy with which the different degrees of illumination belonging to different parts of 
the pencil are represented by the different degrees of brightness of the dispersed light. 
The solution should be contained in a vessel with plane sides of glass, and ought to 
be very weak, or else only the part of the pencil which lies near the surface by whicli 
the light enters will be properly represented. 
Application of internal dispersion to the determination of the absorbing power of media 
with respect to the invisible rays beyond the violet, and the refecting power of surfaces 
with respect to those rays. 
194. Hitherto no method has been known by which the absorbing power of a me- 
dium with respect to these rays could be determined for each degree of refrangibility 
in particular, except that which consists in taking a photographic impression of a pure 
spectrum, the light forming the spectrum having been transmitted through the sub- 
stance to be examined. It is needless to remark how troublesome such a process is 
when contrasted with the mode of determining the absorption which media exercise 
on the visible rays. But the phenomenon of internal dispersion furnishes the philo- 
sopher, so to speak, with eyes to see the invisible rays, so that the absorbing power of 
the medium with respect to these rays may be instantly observed. For this purpose 
it is sufficient to form a pure spectrum, using instead of a screen a highly sensitive 
fluid or solid, such as one of those mentioned in Art. 191, and to hold before it the 
medium to be examined, or else to place the medium over the whole or a part of the 
slit. 
195. In this way the transparency of glass coloured yellow by oxide of silver with 
respect to the violet rays and some of those still more refrangible, which has been 
remarked by Sir John Herschel*, may be at once observed. A set of green glasses 
were found to be very variable in the mode in which they absorbed the invisible rays, 
some absorbing the more refrangible of the rays capable of affecting a dilute solution 
of sulphate of quinine and transmitting the less refrangible, others absorbing the less 
and transmitting the more refrangible, and others again absorbing them all. These 
rays were absorbed by solutions of chromate and bichromate of potash so weak as 
to be almost colourless. A thickness of about a quarter of an inch of sulphuret of 
• Philosophical Transactions for 1840, p. 39. 
3 z 2 
