OBSERVING THE INTERIOR OF THE EYE 685 



its sensations of sight. For luminous appearances may be produced 

 without our perceiving any external object, but merely a part of the 

 eye or an inward irritation ; and yet, in the same manner, we imagine 

 them to be external to ourselves. 



If we shut the eye, and press the head of a pin upon the outer 

 edge of the eyeball, we shall see in the dark field of vision a white or 

 colored spot of light, which has the same form as the compressing 

 body. It will be seen upon the left side of the field if the right side 

 is pressed, and upon the upper half if the lower is pressed, and vice 

 versa. The retina, therefore, extends as far as the part which projects 

 beyond the socket of the eye, and can be irritated by pressure. It is 

 well known that when the eye is struck a cloud of sparks is seen, 

 which is caused by the mechanical concussion of the retina. These 

 luminous images, often perceived involuntarily, take, speaking scien- 

 tifically, the form of the body producing the pressure; at the same 

 time we observe the relation between the position of the irritation 

 and the position of the sensation of sight. We transpose a point on 

 the left side of the retina to the right, because we imagine that a ray 

 of light has penetrated the eye from the right, which must fall upon 

 the left half of the retina. 



We are also able to perceive particles within the interior of the 

 eye which are found in the transparent media. There are many per- 

 sons who always see round particles or filaments, which seem to float 

 about in the field of vision. They may be more distinctly seen when 

 looking upon a bright surface a cloudy sky, or through a microscope. 

 They follow every motion of the eye, and have, moreover, a peculiar 

 motion of their own. These particles are produced by filaments and 

 cells, which may be found floating about in the narrow space between 

 the hyaloid membrane and the retina. They cast their shadow di- 

 rectly upon the retina, which then, from experience, refers them to 

 external objects. 



It has also been discovered by more careful observation that the 

 refracting media of the eye are not absolutely transparent, but that a 

 kind of cloudiness is seen in places which throws a shadow upon the 

 retina. If we look at the sky through a small hole in a sheet of 

 paper, held a short distance from the eye, the hole will appear to be 

 surrounded by a colored fringe. This is caused partly by a cloudi- 

 ness in the vitreous humor, and partly by the peculiar radiating 

 formation of the lens, already described. All such phenomena are 

 called entoptic, because they deal with the perceptions of the internal 

 portions of the eye. They are produced by the incident rays of light 

 casting shadows of these particles upon the retina. They are best 

 seen when an isolated pencil of light, like that admitted through a 

 small aper-ture, is allow 7 ed to fall upon the eye ; for, in that case, the 

 shadows produced are distinct, while they are generally obliterated in 

 ordinary vision, because the light penetrates the eye from all sides. 



