622 PROJECTION EXPERIMENTS IN PHYSICS [Cn. XV 



No directions for the ordinary experiments shown by projection 

 apparatus in every university course in physics and chemistry are 

 given here, but we thought it wise to include a few special projec- 

 tion experiments that have been found by us to be especially 

 instructive or difficult to perform by the means ordinarily used. 



The experiments illustrating normal vision and the simpler re- 

 fractive eye defects are included because of the importance of 

 these defects and their prevalence with school children and stu- 

 dents and others doing close work; and because, with projection 

 apparatus, it is so easy to show in a striking manner what is 

 meant by the defects, and how certainly the defects can be cor- 

 rected by using the proper spectacles. 



SOME SPECIAL EXPERIMENTS IN PHYSICS 



878. Kind of apparatus. Most projection experiments in 

 physics are only shown occasionally, hence permanent apparatus 

 to demonstrate many physical phenomena is so costly as to be out 

 of the question. The apparatus here described consists in lenses, 

 objectives, prisms, clamps, arc lamps, etc., generally to be found in 

 any laboratory where such experiments would be shown. The 

 apparatus can either be clamped to rods or laid upon a horizontal 

 table. The former method has the advantage that one can project 

 in an inclined as well as in a horizontal direction ; the latter method 

 is easier to set up. When permanent apparatus is used the 

 principle is exactly the same, permanent instead of temporary 

 supports being used to hold the lenses. 



An optical bench like that shown in fig. 158-159 is satisfactory 

 where the apparatus is set up on a table. When it must be held 

 at various angles some clamping arrangement is desirable. 



EXPERIMENTS WITH POLARIZED LIGHT 

 879. Apparatus. 



1. Right-angle arc light; Condenser; Water-cell; 



2. Small Nicol prisms with openings of i to 2 cm. which are 

 mounted so that they can be rotated. 



