218 HAND-BOOK OF PHYSIOLOGY. 



Visual Purple. The method by which a ray of light is able to stim- 

 ulate the endings of the optic nerve in the retina in such a manner that 

 a visual sensation is perceived by the cerebrum is not yet understood. It 

 is supposed that the change effected by the agency of the light which 

 falls upon the retina is in fact a chemical alteration in the protoplasm, 

 and that this change stimulates the optic nerve-endings. The discovery 

 of a certain temporary reddish-purple pigmentation of the outer limbs of 

 the retinal rods in certain animals (e.g., frogs) which have been killed 

 in the dark, forming the ^so-called visual purple, appeared likely to oiler 

 some explanation of the matter, especially as it was also found that the 

 pigmentation disappeared when the animal was exposed to light, and re- 

 appeared when the light was removed, and also that it underwent distinct 

 changes of color when other than white light was used. The visual 

 purple cannot however be absolutely essential to the due production of 

 visual sensations, as it is absent from the retinal cones, and from the 

 macula lutea and fovea centralis of the human retina, and does not ap- 

 pear to exist at all in the retinas of some animals, e.g., bat, dove, and 

 hen, which are, nevertheless, possessed of good vision. 



If the operation be performed quickly enough, the image of an object 

 may be fixed in the pigment on the retina by soaking the retina of an 

 animal, which has been killed in the dark, in alum solution. 



Electrical Currents. According to the careful researches of Dewar 

 and McKendrick, and of Holmgren, it appears that the stimulus of light 

 is able to produce a variation of the natural electrical current of the 

 retina. The current is at first increased and then diminished. McKen- 

 drick believes that this is the' electrical expression of those chemical 

 changes in the retina of which we have already spoken. 



VISUAL PERCEPTIONS AND JUDGMENTS. 



Reversion of the Image. The direction given to the rays by their 

 refraction is regulated by that of the central ray, or axis of the cone, 

 toward which the rays are bent. The image of any point of an object is, 

 therefore, as a rule (the exceptions to which need not here be stated), always 

 formed in aline identical with the axis of the cone of light, as in the line 

 of B a, or A 1) (Fig. 381), so that the spot where the image of any point 

 will be formed upon the retina may be determined by prolonging the 

 central ray of the cone of light, or that ray which traverses the centre of 

 the pupil. Thus A I is the axis or central ray of the cone of light issuing 

 from A; B a the central ray of the cone of light issuing from B; the 

 image of A is formed at #, the image of B at a, in the inverted position; 

 therefore what in the object was above is in the image below, and vice 

 versa, -the right hand part of the object is in the image to the left, the 



