27*2 SUPPLEMENT TO THE 



intense at the periphery than any where else, (see figures 

 8 7 and 9) ; it decreases as it approaches the centre, where 

 there remains, however, a strong luminous point, (figure W.} 

 There is no diffusion of light, for the disc is perfectly well 

 defined, and the black ground quite black close to it, con- 

 tinuing so for a long way within the focus. 



On applying Sir D. Brewster's monochromatic lamp, the 

 strong annuli of colour confounded with the spherical aber- 

 ration, disappear, and leave a yellow disc ; a new pheno- 

 menon now becomes visible, before hidden by the chro- 

 matic aberration, viz. the appearance of an infinite num- 

 ber of concentric annuli in the disc of light formed by the 

 globule (figure 8 X ) ; they are perfectly regular, and increase 

 in brightness as they spread away from the centrical point 

 of light. 



On examining the aforesaid objects, in and out of focus, 

 with a spherical metal of the same focus and aperture (used 

 without a plane mirror, by merely placing the dial-plate 

 and star in its focus), the same phenomena, as to spherical 

 aberration, are seen much more distinctly, because, in this 

 case, there is no chromatic confounded with it ; the aber- 

 ration is, however, evidently far fainter. The spherical 

 aberration of metals follows the same law as that of object- 

 glasses, and is represented in figures 3, 4 ; when too sphe- 

 rical, they have the condensation described within their 

 focus ; but when too flat at the edges, or beyond their true 

 figure (which is an ellipsis for the engiscope) for example, 

 supposing them to have become parabolical, hyperbolical, 

 or ultra hyperbolical then their outside rays will become 

 too long ; they may be considered as over corrected, and 

 in this state present the same phenomena as an over cor- 

 rected object-glass ; that is, the principal strengtli and in- 



