PHOSPHENES. 



275 



the dots beside the finger. Move the card outwards, inwards, 

 upwards, and downwards successively, and note that as the 

 dots are moved towards the periphery they appear as one, 

 but not at equal distances from the fixed point in all meri- 

 dians. For convenience, the card may be moved along a rod, 

 movable on a vertical support. 



6. Maxwell's Experiment. The Yellow Spot. 



(a.) Make a strong watery solution of chrome alum filter 

 it, and place it in a clear glass bottle with flat sides. Close 

 the eyes for a minute or so, open them, and while holding 

 the chrome alum solution between one eye and a white 

 cloud, look through the solution. An oval or round rose 

 spot will be seen in the otherwise green field of vision. The 

 pigment in the yellow spot absorbs the blue-green rays, 

 hence the remaining rays which pass through the chrome 

 alum give a rose colour. 



7. Bergmann's Experiment. 



Make a series of parallel vertical black lines, 2 mm. in 

 diameter, on white paper, with equal white areas in- 

 tervening between them. 

 Look at them in a good 

 light, at a distance of two 

 to three yards. In a 

 short time the lines will 

 appear as in Fig. 127, A. 

 They appear of this shape 

 because of the manner in 

 which the images of the fill 

 lines fall on the cones in 

 the yellow spot, as shown 

 in B. 



8. Phosphenes. 



Fig. 127. Bergmann's Experiment. 



(a.) Press the finger firmly, or better still the head of a 

 large pin, against the inner corner of the closed eye. A 

 brilliant circular patch, with a steel-grey centre and yellow 

 circumference, is seen in the field of vision and on the oppo- 

 site side, and of the same shape as the compressing body. 



