OPTICAL PEOJEGTION 



CHAPTER I 



ON PROJECTION 



1. Definition. Projection, in the optical sense and in 

 this book, is the production of a picture or image usually a 

 magnified image upon some kind of screen or visible plane, 

 generally by means of a lens. The image may be that of a 

 picture, or of some solid object or piece of apparatus. It is 

 manifest that the excellence of the projection that is, the 

 brightness and distinctness of the picture on the screen 

 must depend upon many practical conditions, which are to 

 be treated of in these pages. But first of all it is essential 

 that the nature of the problem itself be really understood, if 

 the best result is to be obtained with the apparatus at our 

 disposal. 



2. Image not formed by the Lens. The reader would 

 probably gather from most of the treatises upon optics, that 

 the image on the screen which he has to produce is formed by 

 the lens ; but that is not true, except in a very secondary sense. 

 The first and cardinal truth for an intelligently successful 

 demonstrator to grasp, is the fact that all power of really 

 forming ' an image resides in those rays of light themselves 

 by which any object is visible to us. 



Starting with, and here taking for granted without need- 



