SENSE OF SIGHT. 629 



situated considerably on one side, under a similar degree of divergence, 

 fall upon the cornea and the crystalline lens in such a way that there is 

 more difference in their angles of incidence, and consequently more dif- 

 ference in the amount of their refraction. They are therefore brought 

 together too rapidly, and are dispersed upon the retina over the space 

 ?/, z, forming an imperfect image. Ophthalmoscopic examination of the 

 retina shows that, in point of fact, the images formed at the fundus of 



Fig. 198. 



DIAGRAMMATIC SECTION OF THE EYEBALL, showing difference of refraction for 

 direct and indirect vision. a, x. Rays from a point in the line of direct vision, focussed at 

 the retina. 6, y, z. Rays from a point outside the line of direct vision, brought to a focus 

 and dispersed before reaching the retina. 



the eye, from luminous objects in the line of direct vision, present per- 

 fectly distinct outlines ; while those at a certain distance from this point, 

 toward the lateral parts of the retina, are comparatively ill-defined. 



Secondly, there is reason to believe that the sensibility of the retina 

 is also less acute in its lateral regions than at the fundus, and particu- 

 larly at the macula lutea and fovea centralis; since, according to Helm- 

 holtz, the sharpness of sight for objects at a little distance from the 

 line of direct vision diminishes in greater proportion than the dis- 

 tinctness of their images formed upon the retina. The fovea centralis, 

 according^, is the spot where the retina possesses the most acute sensi- 

 bility, and it is also situated at the extremity of the visual axis, where 

 the refraction and covergence of the luminous rays are effected with the 

 greatest accuracy. Objects situated upon the line of this axis are seen 

 by direct vision, and are distinctly perceived; those situated in the 

 field of view, outside this line, are seen by indirect vision, and their 

 outlines appear more or less confused and uncertain. 



Point of distinct vision, and Accommodation of the eye for different 

 distances. An optical instrument, composed of refracting lenses, can- 

 not be made to serve at the same time for near and remote objects. In 

 a refracting telescope or spy-glass, if the instrument be directed toward 



