W. L. Stevens—Physiological Optics. 399 
teresting facts are also developed as to the relation of special 
fertility to predominance of certain species as determined by 
their particular habit of growth; e. g. whether deep or shallow 
rooted, etc., as well as many other details which can only be 
well appreciated by a careful study of the pages. 
_ The most valuable results obtained are those which show the 
influence of different fertilizers upon the character of the vege- 
tation and the total produce, since they give us a certain clue, 
Teciprocally, to the character and fertility of the soil as indi- 
cated by the vegetation produced thereon, considerations which 
are of importance at the present time in view of the efforts” 
being made to reclaim marsh lands and other areas now of little 
b 
Art. XLVIL—WMr. Backhouse's Observations on Physiological 
Optics ; by W. LEConTvTE STEVENS. 
IN the Introduction to Helmholtz’s Physiological Opties, the 
author states that he was often obliged to desist from his experi- 
Ments on account of the distressing pain caused by them. The 
number of persons who are willing to meet the strain implied 
i a continued series of experiments with their eyes is at best 
small; and therefore I have read with some satisfaction the 
criticism on my work, which has. recently been made by Mr. 
T. W. Backhouse, of Sunderland, England. 
To use the eyes satisfactorily as instruments of research 
Tequires much practice, and Mr. Backhouse’s admissions show 
that he has been laboring under some disadvantages. My own 
Series of papers represented a succession of steps in a prolonged 
Investigation, during which slight modifications had to be made 
Tegarding the relative importance at first attached to some of 
the elements which enter into our visual judgments. ‘ ack- 
house disputes the efficacy of some of my explanations, but in 
©pposition to them I do not find in his paper the record of any 
€xperiments that can be advanced as positive evidence in favor 
of any other explanations. He defends Brewster's geometric 
theory of binocular vision, but apparently without close examin- 
ation of Brewster's own presentation of this. If the theory of 
