274 SUPPLEMENT TO THE 



In focus. The image of the window is seen tinged with 

 orange light, which is encircled with a diffused disc, of a 

 greenish yellow, extending to some distance from it. This 

 halo is surrounded by a margin of a brighter and stronger 

 green, which is succeeded by a blue, indigo, and purple 

 annulet, gradually melting into each other, as in the prism, 

 beyond which the black ground on which the globule of 

 quicksilver was placed, becomes visible. 



Phenomena beyond the focal point. These may be ren- 

 dered sufficiently intelligible by observing that on putting 

 the globule beyond the focus, the colours already enumerated 

 are still visible in the same order and arrangement as 

 before, but fainter, because the discs or halos gradually 

 swell out and become spread over a larger surface, and at 

 length totally disappear in nebulosity. 



Appearances within the focus. On bringing the globule 

 a little within the focus, the warm colours instantly present 

 themselves; thus the image of the window is now of a 

 bluish tint, and around it is a disc of bright yellow, suc- 

 ceeded by an annulet of orange, passing into one of strong 

 crimson, which is again surrounded by nearly the same 

 colours before described as visible at the focal point, viz. 

 borders of bright green, blue, indigo, and purple, blended 

 gradually together. If, however, the globule is brought 

 considerably within the focus, then the following change 

 takes place; the image of the window disappears altogether; 

 in lieu of it is seen a central disc of purple, gradually 

 shaded off into bright blue, which is contained within bor- 

 ders of colour, in the following order green, yellow, 

 orange, and red, which continue in the same state for a 

 long way within the focus, until they disappear altogether. 

 By cutting off the aperture to three-fortieths of an inch, 



