386 PROFESSOR, STOKES ON THE CHANGE OF REFRANGIBILITY OP LIGHT. 
or at least almost rigorously the same, composition throughout, and consists exclu- 
sively of rays less refrangible than h. Consequently, we should have to prepare a 
first medium which was opake with regard to the visible rays less refrangible 
than b, and transparent with respect to the rays, whether visible or invisible, more 
refrangible, and a second medium complementary to the former in the manner 
described in the preceding article. If the pair of media were still strictly comple- 
mentary in this manner, but the point of the spectrum at which the transparency of 
the first medium began and that of the second ended were situated at some distance 
from b, the sensibility of the glass would be exhibited as before, only the maximum 
effect would not be produced, on account of the absorption of a portion either of the 
active or of the dispersed rays, according as the point in question was situated above 
or below b. 
Now, although the commencement of the sensibility of canary glass is unusually 
abrupt, it generally happens that the sensibility of a medium, or at least the main part 
of it, comes on with great rapidity, and lasts throughout the rest of the spectrum, 
though frequently it is most considerable in a region extending not very greatly 
beyond the point where it commenced. In those cases in which the dispersion of 
different tints commenced at two or three different places in the spectrum, I have 
almost always had evidence of the independent presence of different sensitive 
principles, to which the observed effects were respectively due. 
Hence, if we could prepare absorbing media at pleasure, we should get ready for 
general use in these observations a few pairs of media complementary in the particular 
manner already described, but having the points of the spectrum at which the trans- 
parency of the first medium commenced and that of the second ended diffeient in 
different pairs, situated say in the yellow for one pair, in the blue for another, in the 
extreme violet for a third. 
243. It is not of course possible to prepare media in this manner at pleasure, and 
all we can do is to select from among those which occur in nature. Nevertheless it 
is useful, as a guide in the selection, to consider what constitutes the ideal perfection 
of absorbing media for this particular purpose. But before proceeding to mention the 
media which I have found convenient, I will describe the arrangement which I have 
adopted for admitting the light. 
A hole was cut in the window-shutter of a darkened room, and through this the light 
of the clouds and external objects entered in all directions. The diameter of the hole 
was four inches, and it might perhaps have been still larger with advantage. A small 
shelf, blackened on the top, which could be screwed on to the shutter immediately 
underneath the hole, served to support the objects to be examined, as well as the first 
absorbing medium. This, with a few coloured glasses, forms all the apparatus which 
it is absolutely necessary to employ, though for the sake of some experiments it is 
well to be provided also with a small tablet of white porcelain, and an ordinary 
prism, and likewise with one or two vessels for holding fluids. 
