fi* Experiments for investigating 



XII. Of diluting and concentrating the Colours. 



Lifting up or tilting a lens being subject to great uncer- 

 tainty, a surer way of acting upon the colours of the rings 

 is by dilution and concentration. After having seen that 

 very 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 begin to assume a faint red colour. With 5, 6, and 7, 

 this appearance 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 centre 

 of the lens is not kept in a black contact^ which in tjiese 

 experiments 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 di- 

 luted so as to incline to violet, indigo, or blue. With one 

 Qf 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 i 1 feet, orange may be distinguished from 

 the yellow, and indigo from the blue. With 14 feet, some 

 violet becomes visible. When the 122 feet Huygenian glass 

 is laid on a plain slip, and well settled upon it, the central 

 colour is then sufficiently diluted to show that the dark spot, 

 which in small lenses, when concentrated, had the appear- 

 ance of black, is now drawn out into violet, indigo, and 

 blue, with little admixture of green ; and that the white ring;, 

 which used to be about the central spot, is turned partly 

 green with a surrounding yellow, orange, and red-coloured 

 space or ring ; by which means we ^eein to have a fair ana- 

 lysis 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 liti)e side motion at th^ 

 sanie time, and brought the glasses into such contact that 

 they adhered sufficiently to be lifted up together. With 

 this adhesion I perceived a colour surrounding a dark 



centre 



