CHAP. XVII.] ACTION OF THE RETINA IN VISION. 55 



impressions, yet there is good reason to suppose that the hinder part 

 of it is much more capable of appreciating them than the anterior. 

 When using one eye only, we naturally direct it towards the 

 object we wish to inspect, and in that way throw the image 

 to the bottom of the globe. When the eye is thus fixed, 

 objects near the boundary of the field of vision are less distinctly 

 seen than those at its centre. The posterior part of the retina, too, 

 is the best adapted to receive correct images through the dioptric 

 media, and we find its gray nervous layer becoming thinner and 

 thinner towards its anterior border. 



It is probable that the most anterior part of the retina is never 

 used in vision, since it can scarcely receive rays directly from the 

 lens. Dr. Young, by fixing the eye in the most natural direction, 

 viz. forwards and a little downwards, and by then moving before 

 it a luminous object, in various directions until it passed beyond the 

 range of vision, ascertained the range upwards to be 50; downwards, 

 70; inwards, 60; and outwards, 90: the extent in each direction 

 being limited by the contiguous parts of the face. An object, there- 

 fore, occupying only an angle of 120, both in the vertical and hori- 

 zontal direction, and suitably placed, would about fill the field of 

 vision of a single eye, when the organ was fixed as above described. 

 Now, it may be proved that no part of the image of such an object 

 would fall on the anterior part of the retina. 



Perhaps it is only in, or very near, the axis in vision, that sight 

 can be said to be perfect. The existence of the yellow spot of 

 Sffimmerring at that point continues a riddle which the most atten- 

 tive examination of its anatomy has not yet solved. And from the 

 absence of this spot in almost all the lower animals, we are led to 

 doubt its importance to perfect vision. 



To the perfect exercise of vision, as of all the other senses, an 

 effort of attention is necessary; and this effort is naturally accom- 

 panied with a motion of the eyeball towards the object, so that the 

 image may be thrown upon the central part of the retina. The 

 range of motion of the eyeball Dr. Young calculated at 55 in 

 every direction; so that, the head being fixed, a single eye may 

 have perfect vision of any point within a range of 110. This field 

 is further widened by the use of the opposite organ, but beyond 

 this an increased range is only to be acquired by movements of the 

 entire head. 



The internal, or gray nervous layer of the retina seems to be the 

 essential part on which the power of the retina in the process of 

 vision depends. That layer is an unbroken sheet, continuous by its 



