COLOURS OF THICK PLATES. [161] 



In this case the achromatic line of the system is reduced to a point, and the rings are 

 analogous in every respect to the transmitted system of Newton's rings. For the bright ring of 

 the first order R m ± \, and therefore the radius of the ring is equal to 



V fi\ . ch 



which becomes infinite when c = h. Hence if the luminous point be at first situated in front 

 of the eye, and be then conceived to move backwards through the eye till it passes behind 

 it, the rings will expand indefinitely, and so disappear, and will reappear again when the 

 luminous point has passed the eye. 



14. All the preceding conclusions agree perfectly with experiment, so far as qualitative 

 results are concerned ; for I have not taken any measures. The change in the direction of 

 curvature, which I had not noticed till it was pointed out by the formula, may be readily 

 seen, when the flame of a candle is the source of light, by holding the candle at arm's length 

 nearly in front of the eye, but a little to the right, then drawing it back beside the eye, 

 and finally holding it at arm's length behind the head, and as nearly in a line drawn through 

 the eye perpendicular to the mirror as the shadow of the head will allow. 



When the candle is held near the eye, a portion only of the circles can be seen ; the circles 

 are in fact reduced to circular arcs, and these arcs may even become perfectly straight. But 

 when the candle is placed at a good distance from the eye, suppose half-way between the eye 

 and the mirror, and a small piece of card is placed as a screen in front of the flame to keep off 

 the glare of the direct light, the circles, or at least several of them, may be seen complete, 

 except that it must be left to the imagination to fill them up where they are hid by the screen. 

 In this way the manner in which the rings open out from the centre of the circles may be 

 observed, though not for the first ring or two, which open out while the centre is hid by the 

 screen. Instead of a candle with a screen, it is better to use the image of the sun in the focus 

 of a small convex or concave mirror. 



15. The conclusion deduced from theory which was mentioned in Art. 13 cannot, of 

 course, be compared with experiment directly. But the experiment may be successfully 

 performed by substituting for either the luminous point or the eye a virtual image. Using 

 for the luminous point the image of the sun in the focus of a small concave mirror, at the 

 distance of some feet in front of a plane mirror of which the surface had been prepared with 

 milk and water, I placed a piece of plate glass between the mirrors, inclined at an angle of 

 about 45°. The greater part of the light coming from the image of the sun was transmitted 

 through the plate of glass ; and on returning from the large mirror a portion of this light 

 was reflected sideways, so that the rings could be seen by reflexion in the plate of glass with- 

 out obstructing the incident light. The system of rings thus seen was very beautiful, and 

 there was no direct light glaring in the eye, and yet no screen to hide any part of the system. 

 It was easy to know when the image of the eye in the inclined plate lay in a line drawn 

 through the luminous point perpendicular to the plane mirror, by observing when the image 

 of the luminous point seen by reflexion, first at the plane mirror, and then at the plate 

 Vol. IX. Part II. 45 



