368 VISION. LECT. XX. 



determinate kind of sensation only. The retina, upon 

 which images of luminous objects are formed, is less en- 

 ergetically affected by points whose light is less intense ; is 

 more affected by the more illuminated points, and is unaf- 

 fected by the dark points. If images were not formed upon 

 the retina, and if the eye was composed of this membrane 

 only, without the apparatus of lenses, vision could not be 

 distinct: all would be reduced to distinguishing the alter- 

 natives of day and night, of light, of light and darkness. 

 But by means of this apparatus, the action of light is limited 

 to a certain portion of the retina, which exactly represents 

 by its form that of the luminous object. It is, then, an 

 essential condition of vision, that the image should be 

 formed upon the retina, and that the focus of the luminous 

 rays should be bound upon this membrane. I ought also 

 to add, that it has been proved by a curious experiment, 

 for which we are indebted to Mariotte, that vision does 

 not take place with equal distinctness when the image is 

 formed upon different parts of the retina. If we place 

 upon a horizontal black plane [as a sheet of black paper,] 

 and on the same line three small white discs [wafers for 

 example,] 5 or 6 centimetres [about 2 inches] apart, and 

 look at them vertically, in such a position that the eye is 

 distant from them 12 or 15 centimetres [about 5 or 6 

 inches,] and that the nose of the observer is vertical to 

 the middle disk,] then, by shutting one eye, and looking 

 with the other at on^of the lateral discs, the disc placed 

 under the open eye ceases to be visible. This disc be- 

 comes visible by varying the distance at which it was at 

 first placed ; and when this has taken place, if we shut the 

 eye which had previously been opened, and open that 

 which had been closed, and fix it on the middle disc, we 

 shall no longer see that which is verticle to it. The point 

 of the retina oo which is formed the image of the disc, 



