Page:Popular Science Monthly Volume 11.djvu/688

668 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 A. 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-page 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.



HE 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 domestic 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 physician should find something in the public mind to which they can appeal; some little stock of universally acknowledged truths, which may serve as a foundation for their warnings, and predispose toward 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 perhaps an unavowed and half-unconscious, undercurrent of opinion that the phenomena of life are not only widely different, in their superficial characters and in their practical importance, from other natural events; but that they do not follow in that definite order which 