626 EXPERIMENTS WITH POLARIZED LIGHT [Cn. XV 



882. Detecting strain in lenses. Substage condensers from 

 the microscope if free from strain are excellent for showing rings 

 and brushes. The glass of which they are made should be per- 

 fectly homogeneous and should have no effect at all on polarized 

 light. If the glass of which they are made is imperfectly annealed, 

 the lenses will affect polarized light. An achromatic condenser is 

 more likely to be free from strain than an ordinary Abbe condenser, 

 because it is likely to be made of better glass. Strain may be 

 tested for in each lens by holding it between crossed Nicols. A 

 strained condenser if put in position L 2 , will show a black cross on 

 a white ground when no crystal is used. 



883. Setting up the experiment. The condensing lenses LI, 

 and L/2 can be clamped on a ring stand. The distance between 

 them is adjusted until the emerging light is practically parallel, the 

 objective O,is put in place so that the image of the lens L 2 , is in focus 

 on the screen, a Nicol prism N 2 , is put in the narrowest part of the 

 beam of light from the objective. When the analyzer N 2 , is turned 

 to give a dark field on the screen and a crystal section (mica, for 

 example) is placed in the converging light between the two con- 

 densers the field becomes illuminated with bright colors and 

 beautiful patterns. 



The final adjustment of the apparatus is now made, the distance 

 between the condensers is adjusted until the screen has the most 

 uniform light. The objective O, is moved until the figures on the 

 screen are as sharp as possible; the analyzer may also require a 

 slight adjustment. 



884. Brightness. The screen image with polarized light is 

 never very bright, hence a very dark room is needed. A screen 

 picture over one or two meters in diameter, (3 to 6 feet) should 

 never be attempted. 



PROJECTION OF SPECTRA 



885. Apparatus. Magic lantern. 



Stand to hold the objective in proper position if the lantern 

 bed is not long enough. 



Slit with adjustable blades. 



