POLARISED LIGHT. 145 



flaws in the film, which will be observed to run parallel 

 with each other; these flaws should be adjusted at about 

 46° from the square parts of the stage, to obtain the 

 greatest amount of depolarisation. 



"Experiment 3. — If we now take the plate of selenite thus 

 prepared, and place it under the piece of brass on the 

 stage, we shall see, instead of the alternate black and 

 white images, two coloured images composed of the con- 

 stituents of white light, which will alternately change by 

 revolving the eyepiece at every quarter of the circle ; then, 

 by passing along the brass, the images will overlap ; and 

 at the point at which they do so, white light will be pro- 

 duced. If, by accident, the prism be placed at an angle of 

 45° from the square part of the stage, no particular colour 

 will be perceived; and it will then illustrate the phenomena 

 of the neutral axis of the selenite, because when placed in 

 that relative position no depolarisation takes place. The 

 phenomena of polarised light may be further illustrated 

 by the addition of a second double image prism, and a 

 film of selenite adapted between the two. The systems 

 of coloured rings in crystals cut perpendicularly to the 

 principal axis of the crystal are best seen by employing the 

 lowest object-glass." 



To show the phenomena of the rings round the optic 

 axes of the crystals, Mr. Lobb adopts the following plan, 

 which is by far the best, and the rings are exhibited in the 

 greatest perfection : — 



1. The B eye-piece without a diaphragm, and the lenses 

 so adjusted that the field-lens may be brought nearer to, 

 or farther from the eye-lens as occasion may require; thus 

 giving different powers, and different fields, and when 

 adjusted for the largest field it will be full 15 inches, and 

 take in the widest separation of the axis of the aragonite. 



2. A crystal stage to receive the crystals, and to be 

 placed over the eye-piece, so constructed as to receive a 

 tourmaline, and that to turn round. 



3. A tourmaline of a blue tint. 



4. A large Nicol's prism as a polariser. 



5. A common two-inch lens, not achromatic; which 

 must be set in a brass tube long enough when screwed into 



