LIGHT. 109 



NEWTON'S RINGS. 



Choose a piece of white window-glass three or four 

 inches square, and with clothes-pins or other means 

 clamp it to the lens with longest focus you have ; a 

 lens with focal length of two or three feet will answer, 

 though less curvature is better. Find by rocking the 

 lens upon the plate with the thumbs where the point of 

 contact is. This may be seen by a set of rings which 

 surround it, and which move from place to place when 

 the lens is rocked. Having found this place where 

 the rings appear, place it near the focus of the con- 

 denser having a diaphragm of pasteboard with a hole 

 in it not more than a quarter of an inch in diameter 

 just back of the plate. This cuts off most of the 

 light that would otherwise be scattered in the room, and 

 prevents the rings from appearing plain. The objective 

 used may have an inch focus. There will usually 

 be seen as many as six rings, and the outer ones at 

 the distance of twenty feet or more may be two or 

 three feet in diameter. Byinterposing colored glasses 

 or colored solutions, as with the bubbles, these colored 

 rings will appear alternately with black rings. 



RECOMPOSITION OF WHITE LIGHT. 



This may be effected in several ways. 



ist. By receiving the decomposed light from one 

 prism upon the face of another prism like it, but turned 

 so that the ray will have its original direction. 



2d. By a lens. Let the decomposed rays from the 

 prism fall upon a double convex lens placed so near to 

 the prism that all of the colors of the spectrum may 

 pass through it. Bring the screen to the conjugate 



