THE SENSES. 525 



action of the crystalline lens in converging all the rays, emanating from 

 a given point, to a focus at the retina. For this purpose, the density 

 of the lens, the curvature of its surfaces, and its distance from the retina, 

 must all be properly adapted to each other. If the lens were too convex, 

 and its refractive power excessive, or if its distance from the retina 

 were too great, the rays would cross each other and become partially dis- 

 persed before reaching the retina, as in Fig. 133. The visual impres- 

 sion, therefore, of any point in the object, would not be concentrated 

 and distinct, but diffused and dim, from being more or less dispersed 

 over the retina, and interfering with the impressions from other parts. 

 On the other hand, if the lens were too flat, as in Fig. 134, or too near 

 the retina, the rays would not come to a focus, but would strike the 

 retina separately, producing a confused image, as before. In both cases, 

 the immediate cause of the confusion of sight would be the same, 

 namely, that rays from each point of the object are dispersed over 

 the retina ; but in the first instance, this is because they have converged 

 and crossed each other ; in the second, it is because they have only ap- 

 proximated, and have never come to a focus. 



FIG. 133. FIG. 134. 



INDISTINCT IMAGE from excessive refraction. INDISTINCT IMAUE if om deficient refraction. 



The proof that the rays are thus concentrated, in the living eye, at 

 the retina, is furnished by the ophthalmoscope. This instrument con- 

 sists of a mirror, so placed as to illuminate by reflected light, through 

 the pupil, the bottom of the eye under observation, and perforated at 

 its centre by a small opening through which the observer looks. By 

 this means the retina and its vessels, as well as the images delineated 

 upon it, may be seen. According to Helniholtz, luminous objects at 

 a certain distance, when distinctly perceived by the person under obser- 

 vation, present to the eye of the observer well-defined inverted images 

 upon the retina. Furthermore, if, from the eyeball of a recently-killed 

 animal, a circular portion of the sclerotic and choroid be removed at 

 its posterior part, similar inverted images of objects in front of the 

 cornea may be seen by transparency on the exposed portion of the 

 retina. 



It is accordingly certain that divergent luminous rays, in passing 

 through the eyeball, are brought to a focus at the retina, principally by 

 means of the crystalline lens. The formation of a visible image at 

 this spot does not by itself explain the phenomena of vision, since 



