THEORY OF UPRIGHT VISION. FUNCTIONS OF IRIS. .163 



M. Longet explains upright vision by supposing that every 

 external luminous point is felt in the eye according to the 

 direction it takes relative to us. We must, says this eminent 

 physiologist, consider the concave spherical surface of the 

 retina as formed of a mosaic, each elementary part of which 

 is a sort of eye designed to perceive the different luminous 

 impressions in a determinate direction. Every pencil of 

 light emanating from a luminous point, and forming a cone 

 of which the apex and the normal axis correspond to one of 

 these portions, will be perceived in the direction of a line 

 joining the centre of the spherical surface to the object 

 looked at. If we reason in this way for every one of the 

 points which constitute the whole of a visible object, the 

 perception of each of these points will be in the real direc- 

 tion, and that of the whole will also be felt under the same 

 conditions, in regard to the observer. The image therefore 

 is not seen as a complete whole ; each luminous point assist- 

 ing in its formation milking a separate impression on the 

 brain, each one is felt according to the primitive direction of 

 the ray of light, and the whole is seen in its real position. 



Functions of the iris. In order that vision may be distinct, 

 it is necessary that the rays should enter the eye in the 

 direction of what is termed the visual axis, and the various 

 movements of the organ tend unceasingly to place it in a 

 position to fulfil this condition, and it is necessary, also, that 

 the light should be neither too strong nor too weak, and that 

 the rays shall traverse the central portion only of the 

 crystalline, and not its borders. In order to obtain an 

 analogous result in some of their instruments, opticians 

 divide them by means of a partition pierced by a hole in 

 the centre, which is termed a diaphragm. We find a similar 

 arrangement in the eye, " an intelligent diaphragm," to use 

 the expression of M. Longet, that is the iris, which dilates 

 or contracts the pupil in such a manner as to measure the 

 quantity of light necessary to vision, and which only allows 

 those rays to pass which are directed toward the central 

 portion of the crystalline lens. In the dark, or if we look at 

 an object but slightly illuminated, the pupil dilates in order 

 to admit the greatest possible number of rays which are 



