MICROSCOPIC INSTRUMENTS. 365 



the polarizer. Another prism is adapted to the compound 

 body, called the analyzer, of the same construction as the 

 lower one; the conical end to which the object-glasses 

 are usually screwed, is taken away, and another similar 

 piece, having the same screw for the object-glasses, but 

 having a tube entering the compound body in which is 

 contained the prism ; on passing rays from the mirror 

 through the instrument, as arranged with these prisms, 

 and revolving either of the prisms, by turning the stage 

 or the compound body ; it will be found that there are 

 two positions in each revolution in which no light will 

 pass through the prisms, but be completely darkened ; if 

 now a plate of sellenite, or other refracting crystal, be 

 placed on the stage, and be brought into the focus of the 

 object-glass, and the light be brought to pass through the 

 prisms, the sellenite will produce a color according to its 

 thickness ; if one of the prisms be now revolved slowly, 

 we shall find more and more light will be transmitted, 

 but the intensity of the color will be diminished ; and 

 when a quarter of a revolution has been accomplished, 

 the brilliancy of the color will reappear, but Avhat was 

 originally red will become green, and the green will be- 

 come red at a second quarter revolution ; if the sellenite 

 be removed and some very thin crystals of sulphate of 

 copper, tartaric acid, or many other crystalline substances 

 be substituted for it, a most gorgeous set of colors will be 

 seen ; and as the prism is being revolved, the same alter- 

 nations of color will take place as in the sellenite ; every 

 color and every tint of each primary color may be pro- 

 duced by variations of the doubly refracting substances, 

 and the thickness of the same through which the polar- 

 ized light passes. The uses of polarized light in micro- 

 scopic examinations, to their full extent, have not yet 

 been fully developed ; but they are many, and objects 

 that are far too delicate to exhibit any structure under 

 ordinary illumination, will often be well seen under polar- 

 ized light. All structures belonging either to the animal, 

 vegetable, or mineral kingdom, in which the power of un- 

 equal or double refraction is suspected to be present, are 

 those that should especially be investigated by polarized 

 light. 



Microscope, having 3-inch foot or base, 3-inch stage- 

 plate, compound body 8 inches long, fine adjustment 



31* 



