394 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
The reactions of quinine mentioned in my former paper (Arts. 205-208), may very 
conveniently be observed by means of drops of the solution placed on the tablet ; aud m 
this way it is possible to work in a perfectly satisfactory manner with excessively minute 
quantities of quinine. The statement there made, that the blue colour was destroyed 
by hydrochloric acid, &c., must be understood only with reference to the mode of 
observation there supposed to be adopted, which was sufficient for the object m vie^ . 
When the solutions are examined in a pure spectrum formed by sunlight, or^ even 
by the method described in the present paper, it is seen that the blue colour is not 
destroyed by hydrochloric acid, and is even developed to a extent 
on the addition of hydrochloric acid to a previously alkaline solution. Still there i. 
a broad distinction between the two classes of solutions, which is all that is require . 
have since extended a good deal these results, and mean to pursue the subject further. 
Meanwhile I may be permitted to correct an error in Art. 205, relative to tlie e ect 
of hydrocyanic acid, which was there stated to develope the blue colour. The ex- 
periment was made with the acid of commerce, containing a foreign acid, to which 
the effect was probably due. 
Comparison of the relative advantages of diff 'event modes of observation. 
254. At first sight it might have been supposed that daylight could never be more 
than a poor substitute for sunlight in any observations relating to fluorescence. Such, 
however, I consider to be by no means the case. In the first place, when sunlight 
is used it is made to enter a room in a definite direction ; whereas in using absorbing 
media all the rays are employed whose directions lie within a solid angle having t le 
object examined for vertex, and the hole for base. If we leave out the part ot t us 
solid angle which corresponds to trees or houses, the part which corresponds to sk> 
will still be so large as to make up in a good measure for the superior brilliancy ot 
the light of the sun. In the second place, stray light is much more perfectly exclude 
than when a beam of sunlight, containing rays of all kinds, is admitted into a room. 
When indeed the use of sunlight is combined with that of absorbing media, it is 
possible to detect very minute degrees of sensibility. Still for general purposes I 
consider the methods depending upon the use of absorbing media with ordinary 
daylight quite comparable with, if not equal to, those methods involving the use o 
sunlight which are applicable to opake bodies; I allude especially to the method ot 
a linear spectrum. The peculiarities in the composition of the fluorescent light, 
when such exist, can be made out about equally well by both methods. 
But when the substance to be investigated is a solution, or a clear soli o su - 
cient size to be examined as such, methods of observation can be put m puictice uiti 
sunlight which surpass anything that can be done merely by the use of absorbing 
media. In consequence of the absence of stray light, which would otherwise 
dazzle the eye, an amount of concentration of the rays can be brought to bear on 
the object, which enables the observer to detect excessively minute degrees ot sensi- 
bility Thus, when the sun’s light is condensed by a rather large lens, and made to 
