PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 497 
was yellowish green. The dispersion was so copious that when a large lens was used 
the dispersed beam approached to dazzling. The prismatic composition of this beam 
was extremely remarkable. The beam was found on analysis to consist of five bright 
bands, which were equal in breadth and equidistant, or at least very nearly so, and 
were separated by narrow dark bands. The first bright band was red, the second 
reddish orange, the third yellowish green, the fourth and fifth green. I have very 
frequently observed dark bands, or at least minima, in the spectrum resulting from 
the prismatic analysis of dispersed beams, but I have not met with any example so 
remarkable as this, except in a class of compounds which the properties of canary 
glass led me to examine. 
75 . On analysing a beam of sun-light transmitted through a certain thickness of 
the glass, there was found to be a dusky absorption band a little below F, another 
less distinct at FJ G, and the spectrum was cut off a little below G. 
76. When the glass was examined by the third method, the dispersion was found 
to commence abruptly about the fixed line h. It remained remarkably copious 
throughout the whole of the visible spectrum and far beyond, with the exception of 
a band beginning a little above F, and having its centre at about F^ G, where there 
was a remarkable minimum of activity. This band, it will be observed, was situated 
between the bands of absorption already mentioned. The tint of the dispersed light 
appeared to be uniform throughout, except perhaps where the dispersion was just 
commencing. This was the best medium I have met with for showing the fixed lines 
of extreme refrangibility, though some others were nearly as good. 
77- On examining the glass by the fourth method, it was found that the dispersion 
commenced nearly where the dispersed light ended, that is, the lowest refrangibility 
of the rays capable of being dispersed was nearly the same as the highest refrangibility 
of the rays constituting the dispersed beam exhibited by white light as a whole. The 
dispersion appeared indeed to commence a little earlier, at about the refrangibility of 
the fourth dark band in the spectrum of the entire dispersed beam. When the small 
prism was held to the eye with one hand, while the small lens in the board was gra- 
dually moved with the other, in a direction from the red to the violet, through the 
part of the spectrum where the dispersion commenced, it was found that the region 
of the first four bands was lighted up almost simultaneously, the whole field of view 
having been previously dark. When the lens was moved a very little further on the 
dispersed beam with its five bands was formed complete. Indeed the whole five ap- 
peared almost simultaneously. Speaking approximately, and in fact with almost 
perfect accuracy, we may say that if white light be conceived to be decomposed into 
two portions, the first containing rays of all refrangibilities up to that of the fixed 
line h, or thereabouts, and the second containing rays of all greater refrangibilities, 
the dispersed light produced by white light as a whole belongs exclusively to the first 
portion ; and yet, were the bottle illuminated by the first portion alone, no dispersion 
whatsoever would be produced, whereas were it illuminated by the second portion 
3 s 2 
