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 con- 

 struction 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. 



