lS4 OPTICAL PROJECTION 



parallel beam (p. 277). The figure is a ground-plan of the 

 apparatus. As close to the nozzle as convenient are a pair of 

 right-angled prisms, P and p 2 , their hypotenuses or reflecting 

 sides together, and kept close by a rubber band round them 

 near each end ; but a film of air must be preserved between 

 them, which a morsel of tissue-paper will secure if needful. In 

 the paths of both direct and reflected rays are two focussing 



PIG. 150. Newton's Experiment 



lenses F and F, adjusted so as to focus the slit N, and beyond 

 each of these is a prism bottle B or B 2 , adjusted to give a 

 spectrum. They are shown as throwing these on two separate 

 screens at right angles to each other, s s and s s, but it has 

 lately been suggested to me l that by turning the prism B the 

 other way, both spectra would be thrown upon the one screen 



1 By Mr. John Cox, M.A. 



