370 VISION. LECT. XX. 



brain the distinct sensation of a luminous object, not only 

 when its rays are collected into a single one, as when they 

 reach it from a distance of about 30 centimetres [= 11-8 

 inches,] but also when they are collected in a small, very 

 limited, circular space ; or we must suppose, that the cur- 

 vature of the transparent cornea and of the crystalline lens, 

 can be varied for different distances, and that the crystalline 

 lens can change its position ; that is to say, augment or 

 shorten its distance from the retina in different cases. Ac- 

 cording to the calculations of Olbers, it would be requisite, 

 in order to have equally distinct vision at very different 

 distances, namely, from 4 inches to an infinite distance, 

 that the interval between the crystalline lens and the retina 

 could vary at least ^th of an inch, supposing the convexity 

 of the cornea and of the crystalline lens to remain constant. 

 The same result would be obtained, if we were to suppose 

 that the convexity of the crystalline lens and the cornea 

 varied, while the distance of the crystalline from the retina 

 remained the same. Olbers also found, that vision would 

 be distinct within the limits mentioned, if the radius of the 

 cornea were capable of change, to the extent of about the 

 * T 3 7 ths of an inch. 



An experiment made by Scheiner shows, that there are 

 cases in which the image of an object appears to the same 

 eye sometimes double, at others single. If, by means of a 

 needle, we make in a piece of paper two lioles, at a less 

 distance from each other than the diameter of the pupil, 

 and if we look through them with one eye only, at a certain 

 distance the object will appear single; but at a greater or 

 less distance than this, it will appear double. If we close 

 one of the holes, one of the two images will disappear. In 

 the first instance, the two luminous fasciculi evidently meet 

 upon the retina ; while, in the two other cases, the retina 

 is more or less removed from their point of intersection. 



