240 USE OF THE MICROSCOPE. 



the prism, the axes do not correspond; to remedy this, the 

 compound body (if capable of being turned away from the 

 stage, as in Mr. Ross's and Mr. Powell's instruments,) must be 

 shifted either to the right or the left until this point is 

 attained. If now a thin plate of selenite, or other doubly 

 refracting crystal, be placed on the stage, and be brought into 

 the focus of the object-glass, and the light be caused to 

 pass through the prisms, the selenite will produce a colour 

 according to its thickness: if one of the prisms be now 

 revolved slowly, we shall find that more and more light 

 will be transmitted, but the intensity of the colour will be 

 diminished, and when a quarter of a revolution has been 

 accomplished the brilliancy of the colour will re-appear ; but 

 what was originally red wiU become green, and the green will 

 again become red at a second quarter of a revolution. If the 

 selenite be removed, and some very thin crystals of sulphate 

 of copper, tartaric acid, or one of the other substances 

 presently to be enumerated, be substituted for it, a most 

 gorgeous set of colours will be seen ; and as the prism is 

 being revolved, the same alternations of reds and greens will 

 take place as with the selenite. If, however, a piece of glass, 

 with some perfect crystals of iodide of potassium or common 

 salt upon it, be placed under the same conditions, neither the 

 light nor the colours will be seen: hence bodies may be 

 divided into those that polarize and those that do not polarize ; 

 to the latter class belong the iodide of potassium and common 

 salt just named. The primitive form of crystal of these 

 substances is the cube, and it has been found, by experiment, 

 that these crystals, not possessing the property of double 

 refraction, do not exhibit colour when placed between the 

 prisms; but by far the greater number of other crystalline 

 bodies possess this property, which is essential to the production 

 of colour. One of these last must, therefore, be chosen in 

 order to exhibit colours ; but it happens that there is one part 

 or axis of the crystal in which the property of double refraction 

 does not exist ; this is called the axis of [no] double refraction : 

 in some crystals there are two such axes. In other bodies 

 there are certain planes along which, if the refracted ray 



