CAUSE OF ERECT VISION. 



259 



Fig. 117. 



eighteen inches, its outline is distinct and clear ; the rays passing from 

 each point of the object, are brought to a point on the retina, but 

 they reach the retina at different angles ; and, by interposing a card 

 perforated with a single pinhole, the object may be seen by rays, 

 which enter the upper part, or the lower part, or the centre of the 

 pupil. No change, however, in the visual place of the object occurs 

 in this instance, as the card is being shifted ; nor is the image mul- 

 tiplied when seen through several pinholes in the card. 



The last experiment, says Mr. Mayo, proves, that the angle at which 

 rays of light fall upon the retina does not affect our notion of the place 

 of objects; and, taken with the preceding, estab- 

 lishes as an inductive law, that the retina is so 

 constituted, that, however exerted, each point of 

 it sees in one direction only, that direction being 

 a line vertical to it; or that in every instance of 

 vision, each point of an object is seen in the direc- 

 tion of a line vertical to the point of the retina 

 upon which the rays proceeding from it are col- 

 lected. It would seem, however, to be a forcible 

 objection to this view of the subject ; that all the 

 objects, , a f and a" on the line ca", Fig. 117, must 

 fall upon exactly the same point of the cornea ; 

 and, therefore, upon the same point of the retina; 

 yet, as only one of these lines b a is perpendicu- 

 lar to the point of the retina on which the rays 

 are collected, such a perpendicular would obvi- 

 ously refer the position of the object a alone 

 correctly. Moreover, accurate examination would 

 appear to show, that this law of visible direction 

 cannot be optically correct, as the lines of direc- 

 tion cross each other at a point much anterior to T 



. i 11 mi i -, Lines of Visible Direction. 



the centre of the eyeball. This may be proved 



by making a diagram of the eye on a large scale, and laying down the 

 course of the rays entering the organ, according to the curvatures, and 

 refractive powers of its different parts. In this manner, Volkmann 1 

 found, that the lines of direction cross each other at a point a little 

 behind the crystalline, and that they will thus fall at such different 

 angles on different points of the retina, that no general law can be 

 deduced respecting them. 



A certain intensity of light is necessary, in order that the retina 

 may be duly impressed, and this varies in different animals; some of 

 which, as we have seen, are capable of exercising the function of vision 

 in the night, and have hence been termed nocturnal. In man, the 

 degree of light necessary for distinct vision varies according to the 

 previous state of the organ. A person, passing from a brilliantly 

 illuminated room into the dark, is, for a time, incapable of seeing any 

 thing; but this effect differs in individuals; some being much more able 



1 Neue Beitrage zur Physiologic des Gesichtsinnes, Leipzig, 1836, and Muller's Elements 

 of Physiology by Baly, p. 1170, Lond., 1839. See, on this subject, Medical and Physiological 

 Problems, &c., by JX. Griffin, M.D., and Daniel Griffin, M.D., p. 97, Lond., 1845. 



