290 W. LeConte Stevens—Notes on Physiological Optics. 
changes of density are all so many varying degrees of increased 
density, or shall we boldly assume that local expansion is a 
cause of upheaval and the reverse a cause of depression ? 
Mr. Fisher has shown the difficulty which attends the former 
view. There is no difficulty in supposing that the crust and 
subcrust to a depth of 100 to 150 miles (not the true interior 
be it observed) may have contracted its volume. It may have 
sunk and risen again. Calling this ‘columnar 
distinguish it from nuclear contraction, it must be said that 
columnar contraction alone cannot explain the facts. I see no 
resource but to call to our aid columnar expansion. It will at 
once be objected that physical science furnishes us no warrant 
in the known processes of nature for such an assumption. 
Very true. Let us all go to work therefore, and try to find a 
warrant for it. 
Cc. E. DUTTON. 
Art. XXXI.—WNotes on Physiological Optics, No. IL; by 
W. LECoNTE STEVENS. 
1. TuHrory or Assoctatep Muscuntar ACTION. 
IN previous articles’ it has been shown that the current 
theory of binocular perspective applied to the stereoscope 1s 
not only incapable of accounting for many observed facts but 
unsatisfactory even when the visual lines are convergent; 
that the apparent position of points in the stereoscopic field of 
view cannot be determined by any mathematical formula or 
accurately represented by diagram; and that this impossibility 
is due to physiological conditions attendant upon the abnormal 
use of the eyes. 
