512 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
111. An extremely pale solution of nitrate or sulphate of copper is sufficient to 
absorb the extreme red transmitted by a deep blue glass. This is not the case with 
the ammoniacal solution, which does not absorb the extreme red till it is of a pretty deep 
blue. Its absorbing power is greatest, not at the extreme red, but about the orange, 
as may be seen by using candle-light, which is richer in red rays than daylight. 
1 12. Another method of observation which is sometimes useful, consists in employ- 
ing a large lens and absorbing medium, as described in Art. 110, but leaving out the 
additional small lens. The substance to be examined is placed in the condensed 
beam, and viewed through an absorbing medium which is approximately comple- 
mentary to the former. This method is chiefly useful in examining a confused mass 
of various substances. The most minute fragments of sensitive substances show 
themselves in this manner. 
Results obtained with a Linear Spectrum. 
113. When this method is applied to the examination of common objects, it is 
found that the property of producing a change of refrangibility in the incident light 
is extremely common. Thus, wood of various kinds, cork, horn, bone, ivory, white 
shells, leather, quills, white feathers, white bristles, the skin of the hand, the nails, 
are all more or less sensitive. To make a list of sensitive substances would be end- 
less work ; for it is very rare to meet with a white or light-coloured organic sub- 
stance which is not more or less sensitive, I am not now speaking of organic sub- 
stances obtained in a state of chemical isolation, of which some are sensitive and 
others insensible. That substances of a dark colour should frequently prove insen- 
sible is only what might have been expected, because the dispersed light is not reflected 
from the surface, but emanates from all points of a stratum of finite thickness ; and in 
order that dispersed light should be forthcoming, it is necessary that the active light 
entering, and the dispersed light of a different refrangibility returning, should both 
escape absorption on the part of the colouring matter. Such substances usually con- 
sist of a mixture of various chemical ingredients, of which one or more may very 
likely be sensitive, in which case the substance may be compared to a solution of sul- 
phate of quinine mixed with ink. Frequently however the colouring matter is itself 
sensitive. 
114. Among sensitive substances I have mentioned the skin of the hand, which 
stands rather low in the scale. 1 have found the back of the hand a convenient test 
object. When the sunlight is not strong enough to show with ease the derived spec- 
trum in the case of the hand, there is little use in attempting to observe. 
1 15. It is needless to say that papers washed with tincture of turmeric, or with a 
solution of sulphate of quinine, display their sensibility in a remarkable manner when 
examined in a linear spectrum. The sensibility of turmeric paper is rather impaired 
by exposing the paper to the light, but on the other hand is materially increased by 
washing it with a solution of tartaric acid. 
