VIEWING AND ILLUMINATING 



crystal, and being the same in all points of its surface, except upon 

 the little inclined plane which forms its extremity ; but upon oblique 

 portions are seen three or four distinct bands of colour parallel to the 

 edge, and offering to the eye a visible scale or measure of the rapid 

 diminution of thickness in that part. The observed succession of colours 

 in one experiment was the following yellow, brown, purple, blue, 

 sky-blue, straw-yellow, pink, green, bluish green, pink. 



Sulphate of copper, with a drop of nitric aether added to the 

 solution, on a slip of glass, produced minute crystals in the form of 

 rhomboids. These, when placed under the microscope with the field 

 dark, appear like an assemblage of brilliant rubies, topazes, emeralds, 

 and other gems, each being of a different thickness, depolarizing 

 the light in a different degree. If the polarizing eye-piece be 

 now turned a quarter round, the field becomes luminous, and the 

 crystals assume the complimentary colours. Many other salts offer 

 interesting results. Some, however, crystallize in such thin plates, 

 that they do not sufficiently depolarise the light to become visible in 

 the dark ground, such as the minute crystals of sulphate of potash 

 precipitated by aether ; but even these may be often rendered visible 

 when placed on a plate of mica. The beautiful property of dichroism, 

 discovered by Sir D. Brewster, in acetate of copper, may also be 

 exhibited without any trouble, with the polarising microscope. 



Many organic substances appear luminous when the field is 

 darkened, while others are inert, having no sensible action on the 

 polarized light. 



Fragments of coarsely-powdered sugar and of various salts appear 

 more or less bright, and mottled with various colours. Common salt 

 remains dark, and does not act upon the light. 



27. TEST OBJECTS. It is perhaps one of the greatest requisites in 

 the selection of a microscope, to be able to ascertain whether it will 

 be efficient for the purposes intended. This can only be known by 

 its capability of exhibiting those objects submitted to it. Till very 

 recently it was not ascertained that certain objects, in order to render 

 their various markings or texture distinctly apparent, required the 

 instrument to be of the best construction, whether single or compound, 

 and possessing a considerable quantity of distinct light. These objects 



