W, HISLOP OX A NEW ANALYZING SELENITE STAGE. 227 



due to what is called interference of the rays of liglit. This 

 may be briefly described as a splitting and consequent disturbance 

 of the vibrations of the ray, which is effected in the case before us 

 by the introduction of a thin plate of a doubly refracting substance. 

 If I have been fortunate enough to make myself understood, it 

 will be seen that the introduction of the selenite is not merely, or 

 chiefly, to colour the object, but in fact to render evident details not 

 before visible. It will also be readily understood that the polarizers, 

 namely, the prism, the selenite, and the object, should be as close 

 together as possible to get the greatest certainty of result, and to 

 prevent prejudicial effects from other causes. 



But further as to the selenite. I have stated that certain colours 

 are produced by films of a certain thickness, and if a film so pro- 

 ducing a certain colour, say blue, be caused to revolve horizontally, 

 the tint will vary in intensity only ; at one particular position, the 

 least possible trace of colour will be seen. In the ordinary way of 

 arranging the selenites, a film mounted on a glass slide is laid on 

 the stage above or below the object. The effect now produced 

 depends upon the relative position of the axis of polarization of the 

 object, or of its various parts, and that of the selenite. Most proba- 

 bly the various parts of the object have different axes of polariza- 

 tion, and in order to bring them out, the object or the selenite, as 

 well as the polarizer and analyzer, should be capable of rotation 

 while the object is under observation, the object itself remaining 

 stationary. The effect of rotating the selenite will be to bring 

 first one and then another part of the preparation into relief, as 

 shown by the varying tints of colour. 



This effect cannot be produced, nor the object properly examined 

 if the selenites are not capable of rotation. 



In the same way interference may be effected with films of selenite 

 of different thicknesses, superimposed on each other. One will give 

 one particular tint, and if another be superimposed, the tint due 

 to the joint thickness, or to the difference between the two will 

 be given. These films may be capable each of giving a definite colour, 

 but if we place them so that their axes of polarization coincide, no 

 colour will be seen. Eotate either of the films, and its peculiar tint 

 will become visible. Eotate two or more backwards and forwards, 

 and a combination of tints may be obtained, and thus, with 

 three films of selenite, all the colours obtainable by interference can 

 be got in the field of the microscope. 



