6 INTRODUCTION 



1. The optical parts should be arranged on one longitudinal 



axis and fixed in that position, except that the projection 

 objective must be movable along the axis for focusing. 



2. The radiant or source of light should be adjustable in every 



direction to insure proper centering of the light along the 

 optic axis, and to insure the proper relative position of the 

 source of light and the condenser. 



3 . The object carrier for lantern slides is preferably fixed in one 



position; but the stage of the microscope and the object 

 holder of most other kinds of apparatus should be movable 

 along the longitudinal axis so that the object can be put in 

 the cone of light where it will be fully and most brilliantly 

 lighted. 



4. The parts should be constructed so that either (a) it makes 



no difference how they arc placed or (b) so that they can- 

 not be put together wrong. (See footnotes to 4-5, p. 7). 



5. Every part of the apparatus should be dull black to avoid 



reflections. 



Of course for experimental apparatus the more adjustable each 

 part is the greater are its possibilities, but for apparatus to use for 

 definite purposes we believe that no unnecessary adjustments 

 should be possible. 



The custom followed by many manufacturers of sending an 

 illustrated pamphlet giving instructions for installing and using 

 their apparatus, is wholly commendable. In addition it would be 

 advisable in some cases to attach tags to the different parts, stating 

 their purpose and connections (fig. 45). 



All of the apparatus and all of the experiments discussed in this 

 book have been personally tested or observed by us to make sure 

 that they will work; and we have tried to give directions and 

 methods which arc intelligible, and which will most easily produce 

 the desired results. 



Finally, the authors of this book most earnestly advise any one 

 who is to use projection apparatus to go to some place where the 

 facilities arc abundant, and where there is someone skillful in 

 using them. This will give him a standard of what can be accom- 



