118 Mr, Kenfs Experiments with the Prism, [Aug. 



is six inches in diameter, having a focus of two feet three inches ; 

 and I may mention, that I found it requisite that the diameter 

 of the lens should exceed the length of the prism in order to 

 insure a good spectrum. 



Exper. 1. — I threw the colours of the prism on a screen, 

 eleven feet distant, and having placed the lens between them, 

 and only two inches from the prism, 1 found the prismatic 

 colours magnified, and in the same order, to the dimension two 

 feet six inches in width, and one foot three inches in depth. In 

 this case the sun's rays were admitted through a Venetian blind; 

 but when admitted through a hole in a shutter of five inches by 

 four, the dimension was only two feet by nine inches. 



Exper. 2. — Having placed the lens at the distance of two feet 

 six inches from the prism, the figure of the prism was clearly 

 defined, but without exhibiting any prismatic colours whatever 

 on the screen, 



Exper. 3. — I placed the lens three feet from the prism, which 

 produced only the figure of the prism having the violet ray at 

 the bottom, and the yellow above. 



Exper. 4. — When the lens was five feet from the prism, the 

 figure of it was distinctly seen with the prismatic colours 

 reversed. 



Exper. 5. — I placed the lens behind the prism, and threw the 

 sun's rays on it at its focal distance two feet three inches, when 

 the prismatic colours were increased, both in brilliancy and mag- 

 nitude, considerably more than in Exper. 1. 



Exper. 6. — I put the lens within the focal distance of th^ 

 screen, when a small figure of the prism was seen very bright, 

 but without any prismatic colour. 



Exper. 7. — Having placed the lens as in Exper. 2, when no 

 prismatic rays were produced, but a perfect spectrum of the 

 prism in ^ strong white light ; I then placed another prism in 

 the focus of the lens, and to my surprise it produced three 

 colours only, viz. yellow of a greenish tint, red, and deep violet. 

 Wishing to ascertain if those three colours were neutral, I tried 

 them with a third prism, and found not the slightest alteration; 

 and having placed a card so as to receive them, I found, on giv^ 

 ing it a whirling motion, that the colours were entirely lost. 



