470 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
allowed to enter the solid or fluid examined. A coloured glass or other absorbing 
medium was then placed, first so as to intercept the incident rays, and then between 
the substance examined and the eye. For shortness’ sake these positions will be 
designated as the first and the second. Sometimes a coloured glass was allowed to 
remain in front of the hole, and a second glass was added, first in front of the hole 
and then in front of the eye. 
Second Method. — The sun’s light, reflected as before, was transmitted through a 
series of three or four Munich prisms placed one immediately after the other, and 
each nearly in the position of minimum deviation. It was then transmitted through 
a small lens in a board close to the last prism, and so allowed to enter the body to 
be examined, which was generally placed so that the first surface coincided, or nearly 
so, with the focus of the lens. The diameter of the lens was much smaller than the 
breadth or height of the prisms, so that the lens was completely filled with white 
light, the component parts of which however entered in different directions. Re- 
garding the image of the sun in the focus of the small lens as a point, we may con- 
ceive the light incident on the body under examination as consisting of a series of 
cones, corresponding to different refrangibilities, the axes of which lay in a horizontal 
plane and intersected in the centre of the lens, the vertices being arranged in a hori- 
zontal line near the surface of the body examined. 
Third Method. — The sun’s light was reflected horizontally through a vertical slit, 
and received on the prisms, which were arranged as before, but placed at the distance 
of several feet from the slit. A large lens of rather long focus was placed imme- 
diately after the last prism, with its plane perpendieular, or nearly so, to the beam of 
light which had passed through the prisms, and with its centre about the middle of 
this beam. The body examined was placed at the distance of the image of the slit, 
or nearly so. 
Fourth Method. — Everything being arranged as in the third method, a board 
with a small lens of short focus was placed at the distance of the image of the slit, or 
between that and the image of the sun, which was a little nearer to the prisms, inas- 
much as the focal length of the large lens commonly employed, though much smaller, 
was not incomparably smaller than the distance of the lens from the slit. A second 
slit was generally added immediately in front of the small lens. The body examined 
was placed at the focus of the small lens. The dispersed light was viewed from above, 
and analysed by a prism, being refracted sideways. 
The object of these several arrangements will appear in the course of the paper. 
The prisms employed consisted, three of them of flint glass and one of crown. The 
refracting angles of the former were about 43 °, 33 °, and 24 °, and that of the latter 
about 45 °. The refracting faces of the smallest of the prisms (the flint of 43 °) were 
1*35 inch high and 1*60 long. The small lens used was one or other of a pair of which 
the apertures were 0‘34 inch and 0'22 inch, and the focal lengths 0‘75 inch and 0'50 
inch. The focal length of the large lens generally used was about twelve inches. 
