244 PRACTICAL PHYSIOLOGY 



retina, and for different colours. Blue and yellow can be recognised at 

 a greater distance from the fovea than red and green. Still more 

 peripherally all colours appear as black, grey, or white. 



EXPERIMENT I. Milton Bradley Pseudoptics, Section H, Experiment 

 No. 1, conveniently illustrates the variation in the sensibility of the 

 retina for colour. 



EXPERIMENT II. If a perimeter or campimeter be used the 

 boundaries of the field for the different colours can be defined. (See 

 use of perimeter.) 



9. The Perception of Light in different Regions of the Retina. A 

 faint light is often more easily seen when its image does not fall on the 

 fovea, but a few degrees away from this. The recognition of a light at 

 sea on a dark night is often facilitated by directing the gaze some ten 

 degrees to the right or left of the supposed luminous object. Faint 

 stars again may be seen more readily if not directly gazed at. 



10. After-images. After-images may be of two kinds, those which 

 reproduce the original body in all its brightness, those that are the 

 reverse in brightness to the original body. The first are called positive 

 after-images, the second are negative after-images. Positive after- 

 images may be either of similar colour to the original body or comple- 

 mentary in colour, negative after-images are always complementary. 

 They are due to certain changes taking place in the retina and are best 

 observed in the early morning after waking. 



EXPERIMENT I. Close the eyes for two minutes to rest them and 

 then for the briefest possible interval look at some bright source of 

 light as the lamp or the window, closing the eyes again. A bright 

 positive after-image of the source of light will be seen. 



EXPERIMENT II. Look at the incandescent filament through a piece 

 of red glass, as in Experiment I. The positive after-image will appear 

 red. Again look at the filament but for a prolonged period of about 

 half a minute. On closing the eyes the after-image will appear bright 

 but greenish in colour. 



By an alteration of light and dark backgrounds the after-image may 

 be changed from negative to positive. 



EXPERIMENT III. Look at an incandescent lamp for half a minute 

 and so get a well marked after-image. If the eyes be directed to a white 

 surface the after-image will be negative, if to a dark surface it will 

 appear positive. 



[ADVANCED EXPERIMENT. Note the colour of the after-images in 

 Experiment III., and the gradual change in colour which they show. 

 If the after-images tend to fade blink the eyes several times rapidly 

 and they will become more marked. Notice especially the effect of 



