298 OPTICAL PROJECTION 



both away together ; the result of retinal fatigue is now a 

 green disc when the screen is lit up if the glass be blue, 

 the result is an apparently yellow spot. Instead of the open 

 nozzle and loose lens, the aperture and gla.ss may be inserted 

 in the ordinary stage of a lantern with the objective on ; in 

 that case the stage-spring should be held back, in order that 

 the card and glass may be withdrawn instantaneously and 

 without dragging. 



A more beautiful experiment is to project the spec- 

 trum itself on the screen, through a disulphide prism. A 

 rather wide slit should be employed (brilliance more than 

 purity being required) and the 80 must be counted deliberately, 

 so as to give the full time. Also, at the word the nozzle 

 should be covered over, and the gas in the room turned up, 

 the lantern light itself being often rather too brilliant. The 

 complementary spectrum will then appear upon the screen. 



Any light even an Argand burner will perform these 

 instructive experiments ; but it may be well to remark that in 

 every audience there will be a percentage who do not perceive 

 the phenomena, either from colour-blindness, or from unusual 

 endurance of their nervous system, or from not keeping the 

 eye steadily fixed on the index spot, since it is essential that 

 the image on the retina remains fixed in the same place. 



Contrast deceives sensation in' the same way. Arrange a 

 powerful gas-burner (only one) or it may be a less brilliant 

 lantern (as a gas or oil-lantern, used with an oxy-hydrogen 

 one) rather nearer the screen than the lantern, and con- 

 siderably to one side. Throw on the screen a strong light 

 from the experimental lantern, with a coloured glass in the 

 stage or held over the nozzle. After a few seconds hold 

 rods, or a card with pattern-apertures cut in it, in the coloured 

 rays. Where the shadows fall, the screen is free from coloured 

 light, and is illuminated by fainter white light from the other 

 source. But the shadows will appear by contrast strongly 

 tinged of the complementary colour. 



