THE SENSES. 539 



in succession. At the end of a line, it returns suddenly to the next, 

 repeating this movement from the top to the bottom of the page. 



The deficiency of distinctness outside the line of direct vision depends 

 on two causes, both of which contribute to the result, namely : 1st, 

 inaccurate focussing of the rays ; and 2d, diminished retinal sensibility. 



Rays of light entering the eye from the front, in the line of direct 

 vision, are brought to a focus at the retina. But those which enter 

 with a certain degree of obliquity suffer more rapid convergence, and 

 are accordingly brought to a focus and again dispersed before reaching 

 the retina. Thus rays diverging from the point a (Fig. 139), in the 

 line of direct vision, are concentrated at x, and form a distinct image 

 on the retina at that point. But those coming from 6, 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 difference in the amount. of 

 their refraction. They are therefore brought together too rapidly, and 

 are dispersed at the retina over the space y, z, forming an imperfect 

 image. Ophthalmoscopic examination of the retina shows that, in 

 point of fact, images formed at the fundus of the eye, in the line of 

 direct vision, present distinct outlines ; while those at a distance from 

 this point, toward the lateral parts of the retina, are comparatively ill- 

 defined. 



Secondly, the sensibility of the retina is less acute in its lateral 

 regions than at the fundus and the macula lutea ; since according to 

 Helmholtz, the sharpness of sight for objects at a distance from the line 

 of direct vision diminishes more rapidly than the distinctness of their 

 images on the retina. Objects in the visual axis are seen by direct 

 vision, and are distinctly perceived ; those situated within the field of 

 view, but outside this axis, are seen by indirect vision, and appear 

 more or less confused in outline. 



Point of distinct vision, and Accommodation for different distances. 

 An optical instrument, composed of refracting lenses, cannot be made 

 to serve at the same time for near and remote objects. If a refracting 

 telescope or spy-glass be directed toward any part of the landscape, 

 only the objects at a certain distance are distinctly seen ; those within or 

 beyond this distance, are obscure or imperceptible. This is because a 

 m of lenses can bring to a focus at ono point only those rays which 

 strike its surface within a certain degree of divergence. The formation 

 of a visible image at the desired spot depends on the refracting power 

 of the lenses being such, that all rays diverging from the object shall 

 be brought to a focus at the plane where its image is to be perceived. 

 If the object be at an indefinite distance on the horizon, or if it be one 

 of the heavenly bodies, the rays from any point of itz surface reach 

 the telescope under so slight a degree of divergence that they are nearly 

 parallel ; and, on suffering refraction, they will be brought to a focus a 

 short distance behind the lens. But rays emanating from an object 

 less remote, strike the lens under a higher degree of divergence. The 



