226 W. HI8LOP ON A NEW ANALYZING SELENITE STAGE. 



film of Herapathite through which the beam of light is transmitted, 

 or it may be a Nicol's prism constructed of Iceland spar, in a man- 

 ner which I need not now describe. The last three polarizing 

 media are those which are generally used for the microscope, and 

 the Nicol's prism most frequently of all. "Whether the polarizer is 

 a reflector or a refractor, the beam of light is so dealt with that 

 one part is absorbed, and the other part, which is available for illu- 

 mination, is in a modified condition, which is called polarization. 



C is the polarized beam of light; the object is placed at E; 

 at F is the object glass, and above this, either in the body 

 of the instrument or above the eye-piece, is another Nicol's 

 prism or plate of tourmaline, which is called the analyzer. If 

 the object is examined when illuminated by polarized light no 

 effect is observed, except a diminution of light, until the 

 analyzer is applied. If the analyzer or polarizer be now turned 

 round, all the light will be stopped out in a certain relative position, 

 unless the object possesses a doubly refracting structure, when its 

 details will be shown with more or less brilliancy or more or less 

 colour, in accordance with its own peculiarities of structure. A 

 very great number of objects possess this depolarizing power, as it 

 is called, but some in a very feeble degree, and many structures will 

 possess this power unequally, so that one part may be seen while 

 other portions are invisible, until we use additional means of shewing 

 them. This is effected by adding, below the analyzer, but above the 

 polarizer, a plate of selenite (D, Fig. 1), a doubly refracting mineral, 

 selected because of its easily splitting into films. This thin plate 

 of selenite has the effect of assisting depolarization, and thus caus- 

 ing many interesting details to start forth to view, which previously 

 were unseen. 



But it is here necessary to notice that everything depends upon 

 the thickness of the plate of selenite. One thickness will give blue, 

 and upon the revolution of the analyzer or polarizer its complimen- 

 tary colour, yellow, — another will give red and green, and so on. But 

 the mere production of colour is not sufficient. The thickness of 

 the plate of selenite and its consequent effect must be adapted to the 

 depolarizing power of the particular object. This, however, is an 

 unknown element which can only be determined by experiment. 

 Our apparatus should therefore be capable of adjustment as a test 

 or analyzing instrument. 



The effect of a thin film of selenite to which I have alluded is 



