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XVI. On the Change of Ref rangihility of Light.— No. II. By G. G. Stokes, M.A., 
F.R.S., Fellow of Pembroke College, and Lucasian Professor of Mathematics in 
the University of Cambridge. 
Received June 16, — Read June 26, 1853. 
The chief object of the present communication is to describe a mode of observation, 
which occurred to me after the publication of my former paper, which is so con- 
venient, and at the same time so delicate, as to supersede for many purposes methods 
requiring' the use of sun-light. On account of the easiness of the new method, the 
cheapness of the small quantity of apparatus required, and above all, on account of 
its rendering the observer independent of the state of the weather, it might be im- 
mediately employed by chemists in discriminating between different substances. 
I have taken the present opportunity of mentioning some other matters connected 
with the subject of these researches. The articles are numbered in continuation of 
those of the former paper. 
Method of observing by the use of Absorbing Media. 
241. Conceive that we had the power of producing at will media which should 
be perfectly opake with regard to rays belonging to any desired regions of the 
spectrum, from the extreme red to the most refrangible invisible rays, and perfectly 
transparent with regard to the remainder. Imagine two such media prepared, of 
which the second was opake with regard to those rays of the visible spectrum with 
regard to which the first was transparent, and vice versd. It is clear that if both 
media were held in front of the eye no light would be perceived. The same would 
still be the case if the first medium were removed from the eye, and placed so as to 
intercept all the rays which fell on certain objects, which were then viewed through 
the second, provided the objects did nothing more than reflect, refract, scatter, or 
absorb the incident rays. But if any of the objects had the property of emitting rays 
of one refrangibility under the influence of rays of another, it might happen that 
some of the rays so emitted were capable of passing through the second medium, in 
which case the object would appear luminous in a dark field. 
242. Let us consider now how the media must be arranged so as to bring out to 
the utmost the sensibility of a given substance. To take a particular instance, 
suppose the substance to be glass coloured by uranium. In this case the sensibility 
of the medium begins, with almost absolute abruptness, near the fixed line b of 
Fraunhofer, and continues from thence onwards. The dispersed light has the same, 
3 E 2 
