CHAPTER XXX 

 Stimulation of the Retina 



Electrical changes. That an electrical change results from the action 

 of light upon the retina can be shown in the frog. The eye is 

 enucleated and is placed on non-polarisable electrodes, one of which is 

 in contact with the cut optic nerve, and the other with the front of 

 the eye. The electrodes are connected with a galvanometer (or 

 electrometer) (see Chapter XIII.), and the preparation is placed in a 

 dark box. On letting light into the box an electric change is pro- 

 duced in the retina, and the galvanometer is deflected. On removing 

 the light there is another deflection in the same direction. 



Blind spot ; Mariotte's experiment. Make a mark of any sort 

 (such as a cross) upon a piece of paper, and fix one eye say the right 

 upon it, closing the other eye and placing the head about 6 inches 

 from the paper. It will be found that over a considerable area of 

 irregular shape, about 3 or 3 J inches from the cross, the point of a pen 

 or pencil will not be visible because its image falls upon the place 

 where the optic nerve enters the retina. Even a large black dot made 

 in this area upon the white paper is quite invisible as long as the eye 

 is fixed upon the cross. The experiment shows that the optic nerve 

 fibres are insensitive to light. Map out the blind area. 



Macula lutea; Maxwell's experiment. Close the eyes for a minute; 

 then, opening one of them, hold a bottle with parallel sides containing 

 a solution of chrome-alum which has a greenish colour between the 

 eye and a uniform source of light such as a white cloud. The middle 

 of the field of vision will be occupied by an oval rose-coloured area ; 

 the alteration in colour of this part is due to absorption by the 

 yellowish pigment of the macula of some of the rays transmitted 

 through the chrome-alum ; which is dichroic, transmitting greenish- 

 blue and red rays : the former are absorbed. 



Purkinje's experiment. This is to be attempted in a darkened 

 room. Stand about 8 feet from a sheet of white or grey paper fixed 

 to the wall and get an assistant to illuminate the retina through the 

 sclerotic by means of a converging lens and lamp or candle held at 

 one side of the eye. Look steadily at the paper with one eye, accom- 



