INTRODUCTION 



IN THE following pages, Projection Apparatus of various forms 

 and with various sources of light have been considered from 

 a three-fold standpoint: 



(1) The standpoint of the actual user of the apparatus. 



(2) The standpoint of the manufacturer. 



(3 ) The standpoint of the student for whom an understanding 



of the principles involved is of fundamental importance. 



From the first and second standpoints simple "rule of thumb" 

 would answer, and in many cases has answered to bring about 

 fairly good results. For example, the toy magic lanterns so much 

 in evidence at Christmas time, are almost exact copies of the first 

 magic lantern shown by Walgenstein in 1665. The only striking 

 difference is that instead of a candle or lamp without a chimney 

 such as he used, there is now a small petroleum lamp with a glass 

 chimney. 



But for adapting projection apparatus to new conditions and 

 applying it to new uses with the greatest efficiency, the user and 

 the manufacturer must comprehend the fundamental optical and 

 mechanical principles involved. In a word, to make good projec- 

 tion apparatus and to produce good projection in the different 

 fields, the manufacturer and the user must know the principles, 

 and then they must build and must use the apparatus in accordance 

 with those principles. 



Besides the optical and mechanical principles involved in the 

 apparatus, it seems to the authors that the physiology of vision 

 should have prime consideration, because, after all, it is not only 

 the possibility of producing a brilliant screen image that must be 

 thought of, but also the possibility that the observer get a satis- 

 factory impression of that image. With the magic lantern and 

 arc light it is very easy to get screen images as brilliant as daylight 

 scenes in nature. These brilliant images are best seen when the 

 eyes of the observers are adapted to daylight vision. If now, as is 



