APPENDIX II 

 OPTICAL PROJECTION 



IT is not intended in this section to deal exhaustively with 

 the projection of apparatus, etc., as several text-books on 

 the subject exist already, but it has been considered desirable 

 to record the main principles upon which projection depends, and 

 to indicate the most satisfactory way of putting these principles 

 into practice. 



The units of apparatus necessary for the optical projection of 

 slides, apparatus, and experiments, are as follows : 



1. Radiant, or illuminant. 



2. Condenser a lens for the collection of the light rays. 



3. Slide carrier. 



4. Objective a compound lens for the production of the 

 image. 



5. Erecting prism, for erecting the inverted image of 

 apparatus. 



6. Polarizer. 



7. Prism for spectrum. 



It is not necessary for these units to be mounted up in a single 

 appliance called a " lantern," indeed, for many experiments dis- 

 connected units are to be preferred ; but, as a rule, the projecting 

 lantern is the most convenient form of handling the various units. 



i. Radiant. Many illuminating devices are commonly in 

 use, from the incandescent gas mantle to the electric arc, but it 

 will be seen that the most suitable radiant to employ is that which 

 most nearly gives its light from a point, otherwise numerous 

 overlapping images will be produced, and while this is not a great 

 disadvantage in the projection of pictorial slides, such as land- 

 scapes, etc., it is sufficient to render the projection of apparatus, 

 and the production of the parallel beam unsatisfactory, owing to 

 the difficulty of securing precise definition of graduation marks, etc. 



