~13& HERSCHEL ON COLOURED RINGS, 



centrating the by dilution and concentration. After having seen tli at very- 

 colours. Ml 1111 ft 1 • 1 • <* ,i 



small lenses give only black and white when in full contact, 

 we may gradually take others of a longer focus. With a 

 double convex lens of four inches the outward rings will be* 

 gin to assume a faint red colour. With 5, 6, andV, this ap- 

 pearance will increase; and proceeding with lenses of a larger 

 focus, when we come to about 16, 18, or 20 inches, green 

 rings will gradually make their appearance. 



This and other colours come on- much sooner if the ceutre 

 of the lens is not. kept in a black contact, which in these ex- 

 periments must be attended to. 



A lens of 26 inches not only shows black, white, red, and 



green rings, but the central black begins already to be diluted 



so as to incline to violet, indigo, or blue. With one of 34, the 



white about the dark centre begins to be diluted, and shows a 



kind of gray inclining to yellow. With 42 and 48, yellow 



rings begin to become visible. With 55 and 59, blue rings 



show themselves very plainly. With a focal length of 9 and 



II feet, orange may be distinguished from the yellow and 



indigo from the blue. With 14 feet, some violet becomes 



Analysis of the v i s ] D le. When the 122-feet Huygenian glass is laid on a 

 black & white . . ,. s ■ « i , •/ i * ? i i • ;1 ' 



centre. plain slip, and well settled upon it, the central colour is then 



sufficiently diluted, to show that the dark spot, which in smalj 

 lenses, when concentrated, had the appearance of black, is 

 now drawn out into violet, indigo, and blue, with a little ad- 

 mixture of green ; and that the white ring, which used to be 

 about the central spot, is turned partly green with a sur- 

 rounding yellow, orange, and red-coloured space or ring; by 

 which means we seem to have a fair analysis of our former 

 compound black and white centre. 



One of my slips of glass, which is probably a little con<- 

 cave, gave the rings still larger, when the 122 feet glass was 

 firmly pressed against it. I used a little side motion at the 

 same time, and brought the glasses into such contact, that 

 they adhered sufficiently to* be lifted up together. Witli 

 this adhesion I perceived a colour surrounding a dark centre^ 

 A light brown which I have never seen in any prismatic spectrum. It is a 

 kind of light brown, resembling the colour of a certain sort of 

 Spanish snuff. The 170 feet object-glass showed the same 

 colour also very clear! v. 



XIII. 



