200 Dr. Herschei/s Experiments for investigating 
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 center 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 
1 1 feet, orange many 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 appearance 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 seem to 
have a fair analysis of our former compound black and white 
center. 
One of my slips of glass, which is probably a little concave, 
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. With this adhesion I per- 
ceived a colour surrounding a dark center 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 clearly . 
