388 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
also in the case of highly sensitive substances, for the sake of varying the experiment 
and rendering the result more striking. 
As it will be convenient to have names for the media fulfilling these different offices, 
I will call the first medium, or that with which the hole is covered, principal 
absorbent, the second medium the complementary absorbent, and the third medium, when 
such is employed, the transfer medium. For the transfer medium we may choose a 
medium of the same nature as the complementary absorbent, but paler. This is 
perhaps the best kind to employ in the methodical examination of various substances ; 
but if the object of the observer be merely to illustrate the phenomena of the change 
of refrangibility of light, he may vary the experiments by using other media 
245 . I have hitherto spoken only of the increase of illumination due to the sensi- 
bility of the substance under examination. But independently of illumination, the 
colour of the emitted light affords in most cases a ready means of detecting fluores- 
cence. Thus, suppose the principal absorbent to transmit no visible rays but deep 
blue and violet, and the substance examined to appear, when viewed thimugh the com- 
plementary absorbent, of a bright orange colour. Since no combination of the rays 
transmitted by the principal absorbent can make an orange, we may instant y con- 
clude that the substance is sensitive. However, I do not consider it safe, at least for 
a beginner, to trust very much to absolute colour, for few who have not been used to 
optical experiments can be aware to what an extent the eye under certain circum- 
stances is liable to be deceived. The relative colour of two objects seen at the same 
time under similar circumstances may usually be judged of safely enough ; that is, 
of two such colours it may be possible to say with certainty that one inclines more 
to blue or to red than the other. Of course in many cases the change of colour is so 
great that there can be no mistake ; still I think it a safe rule for a person employing 
these modes of observation without having been previously used to optical experi- 
ments, to require some other proof of a change of refrangibility than merely a change 
of colour. Experience will soon show what appearances may safely be relied on. 
246 . If it be desired to view the object isolated as much as possible, it may oe 
placed directly on the shelf, or better still, on black velvet. But it is generally prefer- 
able to have for comparison a standard object which reflects freely the visible rms, 
of whatever kind, incident upon it, and does not possess any sensible dep-ee of fluor- 
escence. It is in this way that the white porcelain tablet is useful ; and m observing, 
I generally place the tablet on the shelf and the object on it. A white plate won d 
answer, but a tablet is better, on account both of its shape and of the compaiat.vt 
dullness of its surface. It is true that the tablet used exhibited a very sensible ainoiin 
of fluorescence when examined in a linear spectrum formed by a quartz tram ; sti 
the effect was so small, and so much of it was due to those highly refrangi e uus 
which are stopped by glass, that for practical purposes the tablet may be regardec 
as insensible. However, an observer is not obliged either to assume that all tablets 
are alike, or to apply to the particular tablet which he proposes to use, methods o 
