PROPERTY OF EMITTING " N RA YS 49 



used for repeating the experiments which show 

 the strengthening of the action on the retina by 

 light when " N " rays are acting simultaneously 

 on the eye. 



These last experiments may be made in a 

 very simple manner. The shutters of a room 

 should be closed so as to leave just enough 

 light for a white surface standing out on a dark 

 background for instance, the dial of a clock 

 to appear, before an observer 4 or 5 metres dis- 

 tant, like a grey patch with ill-defined contours. 

 If a cane stick placed before the eyes is bent, 

 the grey surface is seen to whiten ; if the cane 

 is allowed to straighten, the surface grows dark 

 again. Instead of the cane, a slip of plate-glass 

 can be used. This is bent either with the 

 press employed in lectures for showing the 

 doubly refractive property of glass acquired by 

 flexure, or simply with the hands. With a 

 suitable amount of light, which may be obtained 

 after a few trials, these phenomena are easily 

 visible. They are not instantaneous, as I have 

 already explained. It is of the utmost import- 

 ance that this retardation be taken into account 

 when one wishes to study these phenomena ; 



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