400 W. L. Stevens—Physiological Optics. 
ancestral. 
Mr. Backhouse on p. 306 restates my threefold division of 
the elements of physiological perspective, including that. of 
focal adjustment, and refers in a foot-note to my enumeration 
of the elements that are not physiological, but says that in the 
latter list “focal adjustment is somehow omitted.” Since this 
element is exclusively physiological, the criticism is correct but 
perhaps unnecessary. 
In his observations on the “phantom wall,” the curvature of 
which was noticed but not explained by Brewster, Mr. Back- 
house shows that his negative results are due to the insufficient 
* Seo Edinburgh Transacti vol. xv, Part I . 360. 
+ Brewster on Optics, p. 246. $ ibidem, mem 
