110 PRACTICAL PHYSIOLOGY 



close to the cornea, an erect shadow of the said object will be thrown 

 upon the retina. This, however, will be projected into the field of 

 vision as an inverted image. 



EXPERIMENT. The Experiment No. 6, Section I. in Milton Bradley 

 Pseudoptics, illustrates the nature of retinal shadows well. 



8. The Perception of Colour in the Peripheral Portion of the Eetina. 

 The sensibility of the retina for colour varies in different zones of the 

 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 Eetina. 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. Idio-Betinal Light. Let the eyes be carefully covered, but not 

 pressed on ; let the observer be placed in a perfectly dark room. After 

 a time the effect of recent external light will pass off, but still fine 

 clouds of light may be seen passing over the retina. This is supposed 

 to be due to the action of the blood on the peripheral portion of the 

 visual apparatus. 



11. After-images. After-images maybe 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 



