284 ELEMENTARY PHYSIOLOGY. [XXVII. 



If the movement of the light is stopped, the 



image soon fades away. 



In the first method the image moves in the same 



direction as the light when the light is moved 



from side to side, but in an opposite direction 



when moved up and down, 



In the second method the movement of the image 



is in the same direction as that of the light, 



whether up and down or from side to side. 



12. Look through a microscope with an empty field, 

 illuminated, but not too brightly, by a white 

 cloud, and gently move the head to and fro ; an 

 appearance of fine retinal capillaries will be seen 

 as a dark meshwork on a finely punctated 

 ground. In the centre may be seen a finely 

 punctated spot devoid of capillaries. If the 

 head be moved from side to side, a vertical 

 meshwork will be seen; if up and down, a 

 horizontal meshwork. 



13. The Yellow Spot. Maxwell's Method. 



Place a moderately strong, but perfectly trans- 

 parent solution of chrome alum in a flat-sided 

 glass vessel. Resting the eye for a minute or 

 two, suddenly look through the vessel at a white 

 cloud. A rosy spot or cloud will appear in the 

 centre of vision and remain for some little time, 

 , but will gradually become less distinct. 



The pigment of the yellow spot absorbs the 

 blue-green rays between the lines E and F of 

 the spectrum, these rays removed fi:om those 



