668 THE POPULAR SCIENCE MONTHLY. 



both pieces of glass to the eye, while the reflected images of B and C 

 are, by adjustment, to be made to overlap each other and the image 

 of yl. When this is accomplished a single white object is seen. This 

 experiment is conducted most successfully near a window, and with 

 the back toward the light. When the red and green images are super- 

 imposed a yellow one is seen, and when the green and violet are super- 

 imposed we have blue. The colors originally supposed to be primary 

 were red, yellow, and blue. Prof. Mayer has here given a simple 

 means of refuting this old theory. 



Prof. Mayer describes the construction of a cheap and simple 

 heliostat for directing the sunlight into the room, and keeping the 

 beam in the same position for all these experiments. We refer the 

 reader to the book for the details of its construction, and the full-j^age 

 woodcuts by which it is illustrated. A great point has been gained 

 for scientific education by thus putting it in the power of any student, 

 with ordinary ingenuity, a few tools, and a few shillings, to make such 

 a large number of interesting and instructive experiments. 



--- 



ON ELEMENTARY INSTRUCTION IN PHYSIOLOGY. 



By T. II. HUXLEY, F. E. S. 



THE chief ground upon which I venture to recommend that the 

 teaching of elementary physiology should form an essential part 

 of any organized course of instruction in matters pertaining to domes- 

 tic economy, is that a knowledge of even the elements of this subject 

 supplies those conceptions of the constitution and mode of action of 

 the living body, and of the nature of health and disease, which prepare 

 the mind to receive instruction from sanitary science. 



It is, I think, eminently desirable that the hygienist and the physi- 

 cian should find something in the public mind to which they can ap- 

 peal ; some little stock of universally-acknowledged truths, which may 

 serve as a foundation for their warnings, and predispose tOAvard an 

 intelligent obedience to their recommendations. 



Listening to ordinary talk about health, disease, and death, one is 

 often led to entertain a doubt whether the speakers believe that the 

 course of natural causation runs as smoothly in the human body as 

 elsewhere. Indications are too often obvious of a strong, though per- 

 haps an unavowed and half-unconscious, undercurrent of opinion that 

 the phenomena of life are not only widely difierent, in their superficial 

 characters and in their practical importance, from other natural 

 events ; but that they do not follow in that definite order which char- 



1 A paper read at the Domestic Economy Congress, held in Birmingham, England, 

 July 17-10, 1877. 



