ON THE COLOURS OF THICK PLATES. 181
denote the centre of curvature of the mirror, F its principal focus, L the luminous point, and L3 its image.
Concave mirror: L beyond 0. Eye (1) beyond L\ (2) passing L\ (3) between L and £3; (4) passing Z3; (5) between L3 and the mirror.—Concave mirror: L between 0 and F. Eye (1) beyond L3; (2) passing L3; (3) between Ls and L; (4) passing I; (5) between L and the mirror.—Concave mirror: L between F and the mirror. Eye (1) beyond L; (2) passing L; (3) between L and the mirror.—-Convex mirror. Eye (1) beyond L; (2) passing L; (3) between L and the mirror.
The mirrors employed were formed, as usual, with surfaces of equal curvature. When the observation was made directly, there was no difficulty in determining at which side of the line joining the luminous point and its image the eye lay, and consequently in deciding whether the direction of curvature agreed with theory or not. When the observation was made by reflexion in a plate of glass, the eye was placed so that its virtual imago fell in the line LL^ by moving the head till the image of the luminous point was seen in the centre of the system of rings. The radii of the two surfaces of the mirror being the same, or only differing by a small quantity comparable with the thickness of the glass, the surfaces may be regarded as forming a pair of concentric spheres; and therefore, everything being symmetrical with respect to the line joining L and Zn, when the image of the eye is in this line the bands necessarily become rings, having the image for their centre. Hence the theory of the rings or bands, which it is the object of the experiment to compare with observation, is not involved in the assumption that the image of the eye was in the line LL& when the system of rings appeared arranged symmetrically around the image of the luminous point. By moving the head a little to one side, and observing whether the centre of the system of rings then lay to the right or left of the image, it was easy to compare theory with observation as to the direction of curvature.
There was no difficulty in telling when the virtual image of the eye coincided with the image of the luminous point, since in that case the latter image expanded indefinitely. The phenomena observed offered no direct test of the coincidence of the virtual image of the eye with the luminous point, except what arose from the appearance of the bands themselves. I did not think it worth