14 



OX THE COXSTRUCTIOX OF 



of curvature of the mirror c — the optical axis of the latter being horizontal — and 

 so adjusted that the light which diverges from the illuminated hole in use, may, 

 after impinging on the concave surface of the glass, retiu'n to form an image close 

 by tlie side of the tin screen. In the case of the first test, the returning rays are 

 received into an eye-piece or microscope, d, magnifying 20 times, and moving upon 

 a divided scale to and from tlie mirror. In the second test the eye-piece is removed 

 away from before the eye, and a straight-edged opaque screen, e, is piit in its place. 

 The mirror is supported in these trials by an arc of wood/, lined with thick woollen 

 stuff, and above two Avoodcn latches, </, g, prevent it from from falling forward, but 

 do not compress it. It is, of course, unsilvered. In the figure the table is repre- 

 sented very much closer to the mirror than it should be. In trials on the 151 inch 

 it has to be 25 feet distant. 



The appearance that a truly spherical concave surface presents with the first test 

 is : the image of the hole is sharply defined without any areola of aberration around 

 it, and is surrounded by interference rings. Inside and outside the focus the cone 

 of rays is exactly similar, and circular in section. It presents no trace of irregular 

 illumination, nor any bright or dark circles. With the second test, when the eye 

 is brought into such a position tliat it receives the whole pencil of reflected rays, 

 and the opaque screen is gradually drawn across in front of the pupil, the bright- 

 ness of the surface slowly diminishes, until just as the screen is cutting off the last 



relic of the cone of rays (Fig. 9), the mirror pre- 

 sents an uniform grayish tint, followed by total 

 darkness, and gives to the eye the sensation of a 

 plane. 



If, however, the mirror is not spherical, but 

 instead gradually decreases in focal length toAvard 

 the edge, the following changes result : The 

 image at the best focus is surroimded by a nebu- 

 losity, stronger as the deviation from the sphere 

 is greater, and neither can a sharp focus be 

 obtained nor interference fringes seen. In order 

 to include this nebulosity in the image, it will be necessary to push the eye-piece 

 toward tlie mirror. Before the cone of rays has completed its convergence, the 

 mass of light will be seen to have accumulated at the periphery, and after the focus 

 p. j(j is past and divergence has commenced, the 



accumulation will be around the axis. That 

 is, a caustic (Fig. 10) is formed witli its 

 summit from the mirror. By the second 

 test, in gradually eclipsing the light coming 

 from the mirror, just before all the rays are 

 obstructed, a part of those which have con- 

 stituted the nebulosity wiU escape past the 

 screen (Fig. 11) into the ej^e, and cause 

 there an extremely exaggerated appearance 

 Caustic of Oblate Spheroidal Mirror. in relief of the solid superposed upon the 



Action of the Opariiie Screen. 



