PBOFESSOR STOKES ON THE CHANGE OP REFRANGIBiLITY OF LIGHT. 395 
pass through a strong solution of the ammoniaco-sulphate of copper, the condensed 
beam of violet and invisible rays serves to detect fluorescence in almost all fluids. 
This, however, is no great advantage ; the method is in fact too powerful ; and the 
observer is left in doubt whether the effect perceived be due to the fluid deemed to 
be examined, or to some impurity which it contains in an amount otherwise perhaps 
inappreciable. The great advantage which sunlight observations possess in the 
examination of substances, which, however, is only applicable to clear media, is that 
they enable the observer to make out the distribution of activity in the incident 
spectrum. In some cases this constitutes the chief peculiarity in the mode of fluor- 
escence of a particular substance ; in other cases it enables the observer to see, as it 
were, independently of each other, different sensitive substances existing together in 
solution. Another advantage of sunlight, which applies equally to clear and to 
opake media, is that it enables the observer, with the assistance of a quartz train, to 
make out fluorescence which does not commence till that region of the spectrum 
which it requires a quartz train to show. But such cases are too rare to render 
this a point of much consequence. Of course there are observations such as those 
which relate to the fixed lines of the invisible rays, or to the determination of the 
absorbing action of a medium with regard to invisible rays of each degree of refran- 
gibility in particular, which imperatively require sunlight : I am speaking at present 
only with reference to those observations of which the object is to investigate the 
mode of fluorescence of a particular substance. 
As to the method of observation in which a prism is combined with a principal 
absorbent, its chief use is to determine, in the case of the more sensitive substances, the 
composition of the fluorescent light. It is not generally so convenient as the method 
which involves the use of absorbing media alone for determining which among a group 
of objects are sensitive, and which not, especially when the objects are minute. 
255. Although the description of the mode of observing by means of absorbing 
media has run to some little length, the reader must not suppose that the observa- 
tions are at all difficult. Of course observations of all kinds become more or less 
difficult when they are pushed to the extreme limit of refinement of which they are 
susceptible. But in the case of substances which are at all highly sensitive, and 
this comprises almost all the more interesting instances, the observations are ex- 
tremely easy. I have spoken of a darkened room, which is certainly the most con- 
venient when it can be had. But I have no doubt that an observer who could not 
procure such might easily arrange for himself a darkened box, which would answer 
the purpose. Indeed the fluorescence of highly sensitive substances, though they be 
opake, may be exhibited by means of absorbing media in broad daylight. 
Platlnocyanides. 
256. In the Report of the Twentieth Meeting of the British Association (Edin- 
burgh, 1850, Transactions of the Sections, p. 5), is a notice by Sir David Brewster 
