340 



PRACTICAL PHYSIOLOGY. 



[LXXII. 



Hence, in looking at a large surface, to see it distinctly one must 

 unconsciously move his eyeballs over the surface to get a distinct 

 impression thereof. 



(b.) Make two black dots on a card quite close together, so that when 

 looked at they are seen as two. Hold up the left index-finger, look steadily 

 at it, and place the card with the dots beside the finger. Move the card out- 

 wards, inwai'ds. 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 meridians. For convenience, the card 

 may be moved along a rod, movable on a vertical support. 



6. Clerk-Maxwell's Experiment The Yellow Spot. 



A strong, watery, clear solution of chrome alum is placed in a 

 clear glass bottle with Hat 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 elliptical 

 spot, rosy in colour, 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 intervening between them. Look at them in a good light, 



at a distance of 2 to 3 yards. 

 In a short time the lines will 

 appear as in fig. 258, A. Why? 

 Because of the manner in which 

 the images of the lines fall on 

 the cones in the yellow spot, as 

 shown in B. 



FIG. 258. .bergmann's Experiment. 



8. Phosphenes. 



Press the tip of the finger 

 firmly, or the end of a pencil, 

 against the inner comer 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 

 opposite side. It has the same shape as the compressing body. 

 Press any other part of the eyeball ; the same spectrum is seen, 

 and always on the opposite side. Impressions made on the 

 terminations of the optic nerve are referred outside the eye, i.e., 

 beyond into space. The phosphene is seen in the upper half if 

 the lower is pressed, and vice versd. 



