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4 INTRODUCTION
possible with modern combined apparatus, the brilliant screen image of the transparency is replaced by a relatively dim image projected by the opaque lantern, it will appear exceedingly dim until the eyes can be adjusted to twilight vision. If the operation is reversed after the eyes are adapted to dim light, the brilliant screen image of the transparency will dazzle the eyes.
It is then, not only the dead machine that must be considered, but also the living machine — the eye. It is for the eye that all the work is done, and perfection can be gained only by understanding the workings of the two machines, and adapting the dead machine to the physiologic laws governing the living machine.
Our aim in writing this book then has been to show how good results can be most easily and certainly obtained in all the forms of projection by obeying the laws of physiology as well as those of optics and mechanics.
Naturally, most users of projection apparatus will employ that which is regularly manufactured, but in many institutions not all of the desired apparatus can be afforded. Furthermore, every one who is to do any special work in projection must be capable ot combining and adapting apparatus for those special needs. Hence, we have indicated how home-made apparatus can be got up, and how apparatus designed for one purpose can be utilized for other purposes. We have done this for two reasons, first, because we feel sure that a great gain in efficiency can be made in teaching by the use of the magic lantern, the projection microscope and other forms of projection apparatus, and secondly, because the construction or adaptation of projection apparatus gives one an intimate and working knowledge which more than pays for all the time and trouble.
In examining the apparatus of many different makers we have been impressed with the general excellence of the apparatus and also with certain general defects.
The defects seem to us almost wholly due to the fact that the manufacturers of apparatus and the users of the same are not intimately enough associated, and, therefore, are not so mutually helpful as seems desirable.