194 J. LeConte—Phenomena of Binocular Vision. 
important that it should be so organized), that the perception 
of its images, good or bad, is most perfect in the central spot, 
useless, Per riscopic structure of the lens is useless because periscopic 
perception of the retina is wanting. 
e periscopism, i. e. the power of distinct vision over a wide 
the lens, i.e. of making distinct margin ell as central 
images; 2d, a periscopic property of the retina, i. e. of distinctly 
ceiving marginal as well as central images. ow, Dr. Her- 
mann has left out the second factor, and se identified peri- 
scopism of the lens with periscopism of the 
1 think it quite certain, therefore, that the canes of making 
distinct marginal images, and therefore the characteristic lens 
structure, in so far as 4 sibeerses this purpose, is useless in man. 
Yet it seems equally certain that this property is a very impor- 
tant one. here then shall we seek its use, and what is its 
significance in man? I believe we must seek its use in the 
lower animals, where it originated. In man it must be Pebis 
as an example of a structure which has outlived its usefulnes 
e —_— already shown in our last paper (p. 170), that true 
ism, or distinct vision over a wide field, is a necessar 
siiaiNine of safety in many of the lower animals. In accord- 
ance with the law of evolution, therefore, an ocular structure, 
suitable for this purpose, mus st be gradually formed and per- 
fee This ocular structure, as we have mg seen, consists of 
two parts, a lens structure to produce perfect marginal images 
- a nnd structure to produce e a perception of all images, 
a of these were rs in the 
tribution of rception is inconsistent with fixed attention and 
thoughtful prepton and therefore with the development of 
the higher faculties of the mind. The uses of distinct marginal 
tatigidgien: viz: breadth of distinct view, are sacrificed to the 
igher uses of distinct central perception — viz: fixe 
coougheiat’ attention to the object regarded alone. Thus with 
the development of the central spot of the ser distinct mar- 
ginal perception is lost ; because the uses of the latter are incon- 
sistent with the higher uses of theformer. But the other factor 
of periscopism, viz: the hie of forming distinct margina 
images, is retained by inheritance; because though no longer 
