PROFESSOR STOKES ON THE CHANGE OF REFRANGIBIEITY OF LIGHT. 389 
observation requiring the use of apparatus which he is not supposed to possess. The 
methods of observation described in the present paper are complete in themselves ; 
the observer has it in his power to test for hitnself the tablet he proposes to employ ; 
and he is bound to do so before taking it for a standard of comparison. It may 
easily be tested by means of a prism, as will be explained presently. 
247. The following are the combinations of media which I have chiefly employed : — 
First combination. — In this combination the principal absorbent is a glass coloured 
deep violet by manganese with a little cobalt, or else a glass coloured deep purple 
by manganese alone, combined with a rather pale blue glass coloured by cobalt, or 
with a deeper blue glass in case the day he bright. It is very easy to tell by means 
of a prism, wdth candle-light, whether a purple glass contains any sensible quantity of 
cobalt, on account of the very peculiar mode of absorption which is characteristic of 
this metal. In the examination of substances by this combination no complementary 
absorbent is usually required ; but if it be wished to employ one, a pale yellow glass, 
of the kind mentioned in connexion with the next combination, may be used. 
Second combination. — In this case the principal absorbent is a solution of the 
ammoniaco-sulphate of copper, employed in such thickness as to give a deep blue. 
In my experiments the fluid was contained in a cell with parallel sides of glass, which 
was closed at the top for greater convenience ; but a very broad flat bottle would 
answer as well, because in the case of the principal absorbent the regularity of 
refraction of rays across it is of no consequence. Such a bottle, however, would 
have to be ordered expressly. The complementary absorbent in this combination is 
a yellow glass coloured by silver, and slightly overburnt. These glasses, as com- 
monly prepared, are opakewith regard to most of the violet, but become transparent 
again with regard to the invisible rays beyond ; and, in the case of a pale glass, the 
commencing transparency in the extreme violet may even be perceived by means of 
light received directly into the eye. I have got a glass of a pretty deep orange- 
yellow colour, which is more transparent than common window-glass with regard to 
rays of such high refrangibility as to be situated near the end of the region of the 
solar spectrum which it requires a quartz train to show. But when too much heat 
is used in the preparation, the glass acquires, on the interior of the coloured face, a 
delicate blue appearance, having a good deal of the aspect of a solution of sulphate 
of quinine, though it has in reality nothing to do with fluorescence; and in this 
state the glass is nearly opake with regard to the invisible rays of the solar spectrum 
beyond the violet, though it still transmits a few among those which are nearly the 
most refrangible. Of course, if the complementary absorbent were always left in its 
position between the eyes and the object, its transparency or opacity with regard to 
invisible rays would be a matter of indifference; but as it is desirable that its trans- 
ference from that position to the front of the hole should produce as much difference 
as possible, it is important that it should be opake, or nearly so, with regard to the 
ultra-violet rays transmitted by the principal absorbent. Hence one of these slightly 
