278 ON THE CHANGE OF 11EFRANGIBIL1TY OF LIGHT.
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 T35 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. Once or twice a lens was tried which had a focal length about three times as great, but the light proved too faint for most purposes. In the third method it was sometimes convenient to employ a lens of only 6|- inches focal length, but the 12-inch lens was employed in the fourth method, except on a few occasions, when the lens of 36 inches focal length was used. With the 12-inch lens the length of the spectrum from the fixed line B to H was usually about an inch and a quarter.
It will be convenient for the purposes of this paper to employ certain terms in a particular sense, but as some of these terms relate to phenomena which have not yet been described, it will be well previously to relate in detail what was observed in one remarkable instance of internal dispersion.
Solution of Sulphate of Quinine.
14. The effects of some pale coloured glasses in the case of this fluid have already been mentioned. But there is one glass of which the effect is still more striking. It is well known that a deep cobalt blue glass is highly transparent with regard to the chemical rays. Accordingly I found that a blue glass, so deep that only the brighter objects in a room could be seen through it, produced but very little effect when placed so as to intercept the light incident on the fluid When placed immediately in front of the eye, at first everything disappeared except the light reflected from the convexities of the glass tube; but when the eye became a little accustomed to the darkness it was possible to make out the existence of the band. The contrast between the effects of this glass and of the pale brown glass already mentioned was most striking.
15. When the fluid was examined by the second method, the dispersed light was found to consist of two beams, separated