J. LeConte—Phenomena of Binocular Vision. 191 
termined. Should they prove to have a solid dome they would — 
be included here, and this might detract slightly from the tech- 
nical exactness of the name Paleocrinoidea. Still as its charac- 
teristic types were so prevalent and constituted so important a 
part of the life of Paleozoic ages and the Mesozoic forms are 
comparatively so insignificant in variety and abundance, the 
term would nevertheless be significant and appropriate. 
I shall not attempt to separate the Paleocrinoidee into fami- 
lies, as I think our present knowledge is hardly sufficient for 
such a work, but I feel convinced that it must be based mainly 
upon the diversities in the structure of the vault, not upon the 
construction of the dorsal cup, nor upon the structure of the 
arms or column, upon which former authors have founded such 
divisions, 
The discoveries which have been made within the last few 
, Seopa herein suggested. Other discoveries will follow. 
he labors of the Zoologist will supplement the researches of 
the Paleontologist and through their properly united efforts, we 
may hope in time to comprehend the structure of the Paleocri- 
nidece almost as perfectly as if they were yet living in our 
oceans, 
ArT. XXV.—On some Phenomena of Binocular Vision; by 
JOSEPH LECONTE. 
X. The structure of the crystalline lens and its relation to 
Periscopism.* 
THE following thoughts were suggested by the recent memoir 
of Dr. Ludimar Hermann “On the passage of Juminous pencils 
obliquely through lenses, and on a related property of the crys- 
talline lens of the (human) eye,”+ in connection with my own 
recent publication ‘‘ On the comparative physiology of Binocu- 
lar Vision.” : 
It is well known that the crystalline lens of the mammalian 
eye increases in density and refractive power, from the surface 
to the center; so that it may be regarded as composed of ideal 
* Read before the National Academy of Sciences, April, 1877. 
+ Archives des Sciences, vol. lii, p. 66. t This Jour., vol. ix, p. 168, 1875. 
