PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 511 
peculiarities. If the slit be not too long, the spectrum formed in air is still sufficiently 
pure to allovY us to make out in a general way what are the refrangibilities of those 
portions of the incident light which are most efficient in producing dispersed light ; 
and this is nearly all that can be done even when the spectrum is very pure. 
108. The method of observation which has just been described is that which latterly 
I have almost exclusively employed in examining opake substances. As it will be 
convenient to have a name for it, I shall speak of examining a substance in a linear 
spectrum. In examining substances which are only slightly sensitive, it is often 
highly advantageous to cover the slit with a blue glass. 
109. Fig. 5 is intended to represent the usual appearance of the primary linear 
spectrum, and of the primitive and derived spectra. XY is the primary spectrum, as 
seen by the naked eye, RV, ST are the primitive and derived spectra into which it 
is separated by the prism held to the eye. The direction of the shading in RV is 
intended to represent the composition of this spectrum, which may be regarded as 
consisting of an infinite number of images of the slit arranged obliquely in the order 
of their refrangibility. The direction of the shading in ST is that of the lines of the 
same colour and same refrangibility. Of course the figure does not represent the 
amount of vertical displacement of the primary spectrum when viewed through the 
prism held to the eye. 
110. There is another mode of observation which I have occasionally found con- 
venient when the object was to determine whether a substance exhibited so much as 
a low degree of sensibility. In this method the sun’s light was reflected horizontally 
through a large lens, and then transmitted through a small lens placed in the con- 
densed beam. The small lens was covered by a small vessel with parallel sides of glass^ 
containing a blue ammoniacal solution of copper, or else by a deep blue glass com- 
bined with a weak solution of nitrate or sulphate of copper. The object of the latter 
solution was to absorb the extreme red which is transmitted by a blue glass. The light 
coming through the lens was then analysed by a prism, being received directly into the 
eye, or else allowed to fall on a white object which had been previously ascertained not 
to change the refrangibility of the light incident upon it. I found clean white earthen- 
ware to serve very well for such an object, but each observer ought to test for him- 
self the substance he employs. When a test object, such as white earthenware, is 
used, it is placed at the focus of the lens, and the spot of blue light formed upon it is 
analysed by a prism to see if the absorption is sufficient. When the visible rays are 
considered to have been sufficiently absorbed, the object to be observed is placed at 
the focus of the lens, and the spot of light formed upon it is viewed through a prism. 
The spectrum then seen is compared with that given by the test object. This 
method of observation is rather easier than that of a linear spectrum, and is at least as 
delicate if the object be merely to determine whether a substance is sensitive or not, 
but on the whole it is not near so useful. It may sometimes be used with advantage 
in the case of translucent bodies. 
3 u 
MDCCCLTI. 
