MICROSCOPIC OBJECTS. 129 



field becomes luminous and the crystals assume the com- 

 plementary colours. Many other salts offer interestmg 

 results. Some, however, crystallize in such thin plates 

 that they do not sufficiently depolarize the light to become 

 visible on the dark ground, such as the minute crystals 

 of sulphate of potash precipitated by ether, 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 polarizing 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 hght," 



In the general Catalogue, a number of objects, such 

 as quill, sections of hoof, nail, whalebone, carbonate of 

 lime (animal deposit), &c., will be found interesting for 

 the polarizing microscope. The placing a thin section 

 of selenite or of quartz under the object, as recom- 

 mended in the ' Microscopic Illustrations,' as also the em- 

 ployment of a double image prism for the analyser or 

 upper prism, will often mcrease the variety and beauty of 

 the colours. 



Plants to he viewed hy Polarized Light . — Many plants 

 secrete or deposit on the surface of theu* stems or 



