SENSE ORGANS. 



125 



Fig. 86. — Internal view of the 

 retina, showing the retinal 

 vessels -ramifying over the 

 surface, but avoiding the cen- 

 tral spot. (After Cleland.) 



Experiment 4. Purkinje's Figures. — Darken the 

 room; close one eye, say the left; hold a lighted candle 

 very near the open eye, three or four inches, and to the 

 right side, so that the retina is 

 strongly illuminated. Gaze on 

 the opposite wall until the field 

 of view becomes darkened by 

 excess of light. Now move the 

 candle about, back and forth, 

 up and down. Presently we see 

 a shadowy specter covering the 

 whole wall, like a great bodi- 

 less spider with branching legs, 

 or a spectral tree with leafless 

 branches. What is it? It is 

 an exact but greatly enlarged 

 image of the blood vessels of the retina (Fig. 86). These, 

 ramifying in the granular layer, and therefore in front 

 of the bacillary or receptive layer, cast their shadows on 

 the latter. But any change or variation of this layer is 

 seen as an appearance in space. 



Experiment 5. Ocular Spectra — After-images. — Gaze 

 steadily at the setting sun a moment, and then turn 

 away and look at the wall, the sky, or at a distant build- 

 ing. A colored image follows the eye and is cast on 

 what it looks at. Why so ? The sun's image makes so 

 strong an impression on the retina that it is retained for 

 a considerable time; it makes a brand on the retina. 

 But every change or variation in the retina, whether 

 shadow or image or brand, shows itself as an appear- 

 ance in the field of view. 



We have taken the extreme case of the sun, but any 

 bright object, such as a candle flame in a dark room or 

 a stained-glass window, will produce a similar effect. In 

 the case of bright colors, as in stained-glass windows, 



