INTRODUCTION 7 



plished and what can reasonably be expected. The learner will 

 find that in such a place the apparatus, the room, the screen and 

 the light are all adapted to the purpose to be served. 



Good projection, like any other skilled operation, requires 

 knowledge, facilities and experience. 



There is a very trenchant expression used in shops and in laboratories which 

 seems to us to cover the ground. It is: "Fool Proof." 



From the testimony of many who are especially skilled in machinery and in 

 the use of apparatus, and from our own personal experience, the "fool proof" 

 construction of apparatus is not only necessary for the careless and unskilled, 

 but much appreciated by the most skilled and careful. When one is absorbed 

 in the principles and complexities which some experiment is meant to elucidate, 

 it is a great advantage to have the apparatus which is to be used so constructed 

 that it will go together in the right way with the least conscious effort on the 

 part of the user. The user ought not to be compelled to make a special study 

 of the apparatus every time it is assembled. It is the business of the manu- 

 facturer to put thought into the construction of the apparatus, and it is the 

 user's business to work out problems with it. 



From time immemorial it has been the habit of mankind to make tools, 

 implements and more elaborate apparatus with smooth and glistening surfaces, 

 bright colors often being added to heighten the effect. The microscope and 

 other optical apparatus naturally followed the fashion. 



While to many workers in optics there early came the fundamental apprecia- 

 tion that the clearest images were possible only when absolutely no light 

 reached the eye except from the image field, still polished brass and nickel 

 finish persisted, and the dazzling reflections when bright lights were used, often 

 overwhelmed the image which it was the sole purpose of the apparatus to make 

 visible. 



During the last few years the knowledge of the best conditions for clear 

 images has asserted itself more and more, and the mirror surfaces of optical 

 apparatus have gradually disappeared. At first the dull black apparatus was 

 prepared only for the few who could demand and pay for a special finish. The 

 advantage of the dull finish of optical apparatus is so apparent when once seen 

 and used that now it is becoming very common. 



The great advantage of such dull black, non-reflecting surfaces for the out- 

 side as well as for the inside of optical instruments became apparent to the 

 senior author by the accident of a laboratory fire (1900) during which the 

 lacquer of his best microscope was blackened by the dense smoke. 



The ordinary point of view ten to fifteen years ago that optical apparatus 

 should of course have a 'bright brass or nickel finish is well illustrated by this 

 incident: The senior author was having, by special contract, a microscope 

 with all its accessories made dull black. A visitor, interested in optical goods, 

 going through the factory noticed this lone, black microscope among the 

 brilliant array and asked: "When are you going to bury that one?" 



