168 THE ART OF PROJECTING. 



LESSENING CHROMATIC ABERRATION. 



When a double convex lens is used as an objective 

 as described on page 25, the parts of the picture upon 

 the screen near the margin of the disc will be seen to 

 have many of the lines brightly colored with spectrum 

 tints. At a distance of fifteen or twenty feet from the 

 screen these spectral colors do not give much trouble, 

 but nearer they are oftentimes objectionable. By using 

 a compound objective, such as is made for lanterns, and 

 especially for photographic work, all this may be 

 avoided ; but such compound objectives cost considera- 

 ble. If one cannot afford such a lens, he can use two 

 similar lenses having the same or nearly the same focal 

 length. Use one of these for a condenser, placing the 

 picture close to it, and permit the converging rays to pass 

 through the middle of the lens used as an objective. 

 The trouble willlargely.be prevented, especially if the 

 objective be covered, except a round or square hole at 

 its middle, so that no light will pass to the screen except 

 what goes through the orifice and the middle of the 

 lens. A plano-convex lens two or three inches in di- 

 ameter, and with a focus of eight or ten inches, so used, 

 will do as well as a combination achromatic costing ten 

 or twenty times as much. For microscopic projections, 

 small objectives, such as are used for taking multiple 

 tin-types, answer nearly as well as the more costly ones. 

 They may be had with ratchet movements for about 

 five dollars, and without the ratchet for much less. 

 Their focal length is about an inch. 



BUBBLE COHESION. 



A group of soap bubbles in contact with each other 

 cohere together, and their surface tension always 



