SENSE OF SIGHT. 615 



the whole of the paper will be moderately and uniformly illuminated by 

 the diverging rays. But if a double convex glass lens, with suitable 

 curvatures, be interposed between the paper and the light, the outer 

 portions of the paper will become darker and its central portion brighter, 

 because a portion of the rays are diverted from their original course and 

 bent inward toward each other. By varying the position of the lens 

 and its distance from the paper, a point will at last be found, where 

 none of the light reaches the external parts of the sheet, but all of it is 

 concentrated upon a single spot ; and at this spot will be seen a distinct 

 inverted image of the end of the candle and its flame. 



Distinct perception of the figure of external objects thus depends 

 upon the action of the crystalline lens in converging all the rays of 

 light, emanating from a given point, to an accurate 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 con- 

 verge to a focus too soon, and would not reach the retina until after 

 they had crossed each other and become partially dispersed, as in Fig. 

 192. The visual impression, therefore, coming from any particular point 

 in the object, would not be concentrated and distinct, but diffused and 

 dim, from being dispersed more or less over the retina, and interfering 

 with the impressions from other parts. On the other hand, if the lens 

 were too flat, as in Fig. 193, or placed too near the retina, the rays 



Fig. 192. Fig. 193. 



INDISTINCT IMAGE from excessive INDISTINCT IMAGE from deficient 



refraction. refraction. 



would fail to come together at all, and would strike the retina sepa- 

 rately, producing a confused image, as before. In both these cases, the 

 immediate cause of the confusion of sight is the same, namely, that 

 rays coming from the same point of the object strike different points of 

 the retina ; but in the first instance, this is because the rays have actually 

 converged and crossed each other ; in the second, it is because they have 

 only approximated, but have never converged to a focus. 



The proof that the rays emanating from luminous objects are ac- 

 tually thus concentrated, in the interior of the living eye, upon the 

 retina, is furnished by the use of the ophthalmoscope. This instrument 

 consists essentially of a mirror, so placed as to illuminate by reflected 

 light, through the pupil, the bottom of the eye which is under observa- 



