FORMATION OF IMAGES IN THE EYE. 791 



Formation of Images in the Eye. 



It is only necessary to call to mind the general arrangement of the different structures 

 in the eye and to apply the simple laws of refraction, to comprehend precisely how images 

 are formed upon the retina. 



The eye corresponds to a camera obscura. Its interior is lined with a dark, pigment- 

 ary membrane (the choroid), the function of which is to prevent the confusion of images 

 by internal reflection. The rays of light are admitted through a circular opening (the 

 pupil), the size of which is regulated by the movements of the iris. The pupil is contracted 

 when the light striking the eye is intense and is dilated as the amount of light is dimin- 

 ished. In the accommodation of the eye, the pupil is dilated for distant objects and con- 

 tracted for near objects ; for, in looking at near objects, the aberrations of sphericity and 

 achromatism in the lens are more marked, and the peripheral portion is cut off by the 

 action of this movable diaphragm, thus aiding the correction. The rays of light from an 

 object pass through the cornea, the aqueous humor, the crystalline lens, and the vitreous 

 humor, and they are refracted with so little spherical and chromatic aberration, that the 

 image formed upon the retina is practically perfect. The layer of rods and cones of the 

 retina is the only portion of the eye endowed directly with special sensibility, the impres- 

 sions of light being conveyed to the brain by the optic nerves. This layer is situated 

 next the choroid, but the other layers of the retina, through which the light passes to 

 reach the rods and cones, are perfectly transparent. 



It has been positively demonstrated that the rods and cones are the only structures 

 capable of directly receiving visual impressions, by the following interesting experiment, 

 first made by Purkinje: We concentrate upon the sclerotic, with a convex lens of short 

 focus, an intense light, at a point as far as possible removed from the cornea. This passes 

 through the translucent coverings of the eye at this point, and the image of the light 

 reaches the retina. If we then look at a dark surface, we have the field of vision present- 

 ing a reddish-yellow illumination, with a dark, arborescent appearance produced by the 

 shadow of the large retinal vessels ; and, as we move the lens slightly, the shadow of the 

 vessels moves with it. Without going elaborately into the mechanism of this remarkable 

 phenomenon, it is sufficient to state that Heinrich Miiller has arrived at an absolute mathe- 

 matical demonstration that the shadows of the vessels are formed upon the layer of rods 

 and cones, and that this layer alone is capable of receiving impressions of light. His ex- 

 planation is accepted by all writers at the present day and is regarded as positive proof 

 of the peculiar sensibility of this portion of the retina. In carefully-conducted observa- 

 tions of this kind, a spot is seen in which no vessels appear, which corresponds to the 

 fovea centralis. When the experiment is prolonged, the vessels disappear, as the sensi- 

 bility of the retina becomes diminished by fatigue. 



Theoretically, an illuminated object placed in the angle of vision would form upon the 

 retina an image, diminished in size and inverted. This fact is capable of actual demon- 

 stration by means of the ophthalmoscope. With this instrument, the retina and the im- 

 ages formed upon it may be seen during life with perfect distinctness. 



All parts of the retina, except the point of entrance of the optic nerve, are sensitive to 

 light ; and the arrangement of the cornea and pupil is such, that the field of vision is, at 

 the least estimate, equal to the half of a sphere. If a ray of light fall upon the border of 

 the cornea at a right angle to the axis of the eye, it is refracted by its surface and will 

 pass through the pupil to the border of the retina upon the opposite side. Above and 

 below, the circle of vision is cut off by the overhanging arch of the orbit and the malar 

 prominence ; but externally the field is free. With the two eyes, therefore, the lateral 

 field of vision must be equal to at least one hundred and eighty degrees. It is easy to 

 demonstrate, however, by the ophthalmoscope, as well as by taking cognizance of the 

 impressions made by objects far removed from the axis of distinct vision, that images 

 formed upon the lateral and peripheral portions of the retina are confused and imperfect. 



