PROFESSOR STOKES ON THE CHANGE OF REFRANGIBIJHTY OF LIGHT. 5^9 
little below the plane of the surface, there enters it, not only the light coming directly 
from the blue stratum itself, but also that coming from its image formed by total 
internal reflexion. This mode of observation has already been employed by Sir John 
Herschel in the case of sunlight. As it is frequently useful in these researches it 
will be convenient to have a name for it, and I shall accordingly speak of it as the 
method of observing by superficial projection. 
199. The opacity of a solution of sulphate of quinine appears to increase regularly 
and rapidly with the refrangibility of the light. Hence we may forcn an estimate of 
the refrangibility of any light by which the solution may be affected, by observing 
the degree in whieh the illumination is concentrated in the neighbourhood of the 
surface. For this purpose it is essential to employ a weak solution, since otherwise 
streams of invisible light of various degrees of refrangibility produce each their full 
effect in strata so very narrow, that they cannot be distinguished by the breadth of 
the stratum. Now to judge by the great concentration of the illumination produced 
by a spirit-lamp, even in the case of an extremely weak solution, as well as by the 
considerable degree in which the active rays were intercepted by glass, these rays, 
taken as a whole, must have been of very high refrangibility, such as to place them 
among the most refrangible of the fixed lines represented in the map, or perhaps 
even altogether beyond them. In making observations on the solar spectrum, it 
was plain that the prisms were by no means transparent with respect to the rays 
belonging to the group p of fixed lines. Yet these rays, before they produced their 
effect, had to pass twice through the plate-glass belonging to the mirror (except so 
far as regards the rays reflected at the first surface), then through three prisms, 
though to be sure as close as possible to the edges, then through a lens by no means 
very thin, and lastly, through the side of the vessel containing the fluid. Such a 
train of glass would be sufficient materially to weaken, if not even wholly to cut off' 
the active rays coming from the flame of a spirit-lamp. 
200. The flame of naphtha produces nearly the same effect as that of alcohol. The 
flame of ether is not so good ; but whether this arises solely from its richness in 
visible rays, which only produce a glare, or likewise from a comparative poverty in 
highly refrangible invisible rays, it is not easy to say. The flame of hydrogen pro- 
duces a very strong effect. The invisible rays in which it so much abounds, taken 
as a whole, appear to be even more refrangible than those which come from the 
flame of a spirit-lamp. In making some observations with the flame of hydrogen, 
when the gas was nearly exhausted, so that the flame was reduced to a roundish 
knob no larger than a sweet pea, and giving hardly any light, it was found still to 
produce a very marked effect when held over the surface of a solution of sulphate of 
quinine. The flame of sulphuret of carbon produces on most objects a much stronger 
effect than that of alcohol. It exhibits distinctly the blue light dispersed close to the 
surface of a solution of guaiacum in alcohol, which the flame of alcohol does not .It 
appears then that the flame of sulphuret of carbon is rich in invisible rays of such u 
