LIGHT. loi 



to project a rainbow with an artificial shower, although 

 it can be done by having the beam a widely diverging 

 one, to fall upon spray from a small fountain that 

 spreads a thin sheet at right angles to it ; but a bow 

 that will rival the natural one in the sky may be pro- 

 jected by using two lenses with short focus, such as 

 are put into lanterns for condensers. 



Fig, 77, 



Place the first lens in position, as if for common 

 projection. If the second lens be now brought near 

 the focus of the former and slowly moved towards the 

 screen, a luminous disk will appear upon it, having a 

 red border. Let this disk be made as large as is 

 desirable, which can be done by moving the lens back- 

 ward or forward. Now cut a piece of paper r, with 

 a round top a little smaller than the diameter of the 

 lens, and place it at r. All the light from the lower 

 part of the screen will be cut off, and nothing will be 

 left but a bow, with the colors in the same order as 

 those in the primary bow, and very brilliant. It may 

 be enlarged to twenty or thirty feet. 



A second method requires a conical prism ; and, if 

 this is not already possessed, one may be made by 

 taking a thin, clear, glass funnel, with a mouth three 

 or four inches in diameter. Cut a piece of plain white 



