14 



ON THE C X S T R U C T I N OF 



Fig. 9. 



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

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

 after impinging on the concave surface of the glass, return to form an image close 

 by the side of the tin screen. In the case of the first tt>st, the returning rays are 

 received into an eye-piece or microscope, (/, magnifying '20 tim(>s, and moving upon 

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

 away from befon; the eye, and a straight-edged opaque screen, e, is put in its place. 

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

 stuff, and a])ove two Wooden latches, ;/, </. ]n-event it from from falling forward, but 

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

 sented very much closer to tlie luiiror than it should l)e. In trials on the 151, inch 

 it lias to be 25 feet distant. 



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

 is: the image of the liole 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 traci- of irregular 

 illumination, nor any brigbt or dark circles. \\ ith the second test, when the eye 

 is l)rouglit into such a position that it receives tlie whole pencil of refiected rays, 

 and the opa(|ue screen is gradually drawn across in front of tlie ])upi], tlie bright- 

 ness of the surface slowlv diniinishes, tuitil just as the screen is cutting ott' the last 



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

 sents an uniform grayish tint, followed l)y total 

 darkness, and gives to the eye the sensation of a 

 plane. 



If, however, the mirror is not s])herical, hut 



instead gradually (Jtrrca.seN in focal length toward 



the edge, the following changes result : The 



image at the best focus is surrounded by a nel)n- 



losity, stronger as the deviation from tlie sphere 



is gr(>ater, and neither can a sharj) focus be 



obtained nor interference fringes seen. In order 



to include tliis neludosity in the image, it will be necessary to push the eye-piece 



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



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



is past and divergence has commenced, the 

 accumulation will be around the axis. That 

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

 summit from the mirror. By the second 

 test, in gradnallj- eclipsing the light coming 

 from the mirror, just before all the rajs are 

 obstructed, a part of those which have con- 

 stituted the nebulosity will escape past the 

 screen (Fig. 11) into the eye, and cause 

 there an extremely exaggerated appearance 

 Caustic of Oblate Spheroidal Mirror. In relief of the solid superposed upou the 



Action of the Opaijue Screen. 



Fig. 10. 



