170 J. LeConte on Binocular Vision. 
margins of the visible field, where it is least. Now as the retina 
corresponds with the visible field point for point, it follows 
that there is a corresponding graduation, certainly in sensitive- 
ness, probably in fineness of organization of the receptive layer 
of the retina, from the central spot even to the extreme anterior 
margin. Except in the central spot this increasing fineness of 
organization has not been demonstrated, but it doubtless exists, 
for the increasing distinctness of vision toward the point of 
sight is its effect and its representative in the field of view 
As we go down the vertebrate scale, the central spot is found 
only insome monkeys. After a total absence in other mammals 
and in all birds, it is said to reappear in some lizzards; but 
whether it there has the same structure, and therefore the same 
significance, as in the human eye is not certain. It seems fair 
to conclude, therefore, that the graduation of distinctness toward 
the point of sight and the limitation of the greatest distinctness 
to that point, do not exist in most of the lower animals. 
e importance of the central spot in the highest animals, 
and especially in man, is verv evident. The limitation of the 
greatest distinctness to the point of sight is absolutely necessary 
to the concentration and limitation of the most thoughtful attention 
to that point. If all portions of the retina were similarly or- 
ganized, and therefore all points in the field of view equally dis- 
tinct, it would be impossible to fix the attention steadily and 
thoughtfully on any one point to the exclusion of others. We 
might see equally well and over a wider field, but we could not 
look attentively. Wecould not observe thoughtfully. But in the 
lower animals, especially those which are preyed upon by others, 
it is far more important to see well in every direction than to 
fix the attention too exclusively on one point; the advantages 
of exquisite shat distinctness of the center of the field 1s 
sacrificed for the much greater advantages of moderate distinct 
ness over a very wide field. The most important thing for them 
is a very wide field, and the equal distribution of attention over 
every part. Hence their eyes are prominent, set wide apa! 
on the margins of the front or on the sides of the head, and desti- 
tute of central 2 so that they sweep the whole horizon and 
ith nearly equal distinctness. : 
The connection of the central spot with binocular vision : 
difficulty in the way of oo determination of the 
horopter, as already explained in one of my early papers.* 
* This Jour., IT, vol. xlvii, p. 153. 
