OUR VISION. 



103 



albino's; that is to say, ^the choroid contains no black pigment, but a trans- 

 parent matter, and when placed before a brilliant object, the image can be 

 seen inverted on the retina. The experiment succeeds also with the eye of 

 a sheep or a cow, if the sclerotic has been lessened. The optic centre of the 

 eye is the point where the secondary axes cross ; the optic axis passes through 

 the geometrical axis of the organ, and directs itself spontaneously towards 

 the point that attracts the eye. 



We will now point out in what distinct vision consists. A screen 

 placed behind a lens will only receive the image of a lighted object, A B, if 

 placed in a position, RR (fig. 96). If 

 placed nearer at R" R", or further off at 

 R'R', the light from the object is thrown f - 7/ * 

 on the screen, and the image is confused. 

 To prove the imperfection of sight which 

 is shown by the application of these theo- 

 retic rules, MM. Boutan and d'Alm&da * 

 cite the following experiment : If the -Diagram of mode of vision. 



head of a pin is placed from one to two inches from the eye, nothing 

 will be perceived but a confused haziness of vague outline. The distance 

 of distinct vision is that at which an object of small dimensions may be 

 placed to be plainly perceived. This distance, which averages fifteen inches, 

 varies with different individuals. It can be determined for different sights by 

 means of an apparatus constructed by Lepot. A white thread, a, is stretched 

 horizontally on a dark board (fig. 97). We look at it by placing our eye 

 at one end behind a little screen pierced with an aperture, O ; it then appears 

 much reduced in length, but either nearer or farther off it seems to enlarge 

 and swell, having the appearance of a white surface, becoming larger and 

 larger in proportion as we move away 

 from the point at which it is seen most 

 distinctly. In this manner we can 

 easily obtain a measure of the distance 

 of distinct vision. One of the most 

 remarkable properties of the eye con- 

 sists in the faculty which this organ 

 possesses of seeing different distances, 

 chamber, there is but one distance at which an object will be perfectly 

 visible ; nevertheless a metal wire, for example, can be seen as well at a 

 distance of seven, as ten, fifteen, or twenty inches by good sights. 



This faculty of accommodation in the eye is thus demonstrated : we 

 place two pins, one in front of the other, one eye only being open ; .we first 

 look at the nearest pin, which appears confused if it is near the eye, but by an 

 effort of will the image becomes clear. If, while preserving the clearness of 

 the image, we then carry our attention to the second pin, we find that it, too, 

 presents a confused appearance. If we make an effort to distinguish the 



* Traitt de Physique, Paris 1874. 



It we consider it as a dark 



