128 MICROSCOPIC OBJECTS. 



colour when viewed in considerable thicknesses, is white 

 and transparent when it is extremely thin, and its crystals 

 can be procured so small as to be quite destitute of j)er- 

 ceptible colouration. A drop of it was placed upon a 

 warm piece of gla§s and suffered to evaporate gradually. 

 The crystals, shooting out from the edge of the drop 

 into the interior of the liquid, had a long and narrow 

 rectangular form with a slanting extremity, which may 

 be compared in shape to the straight edge of a chisel. 

 Seen by common light these crystals offer nothing 

 peculiar, but on the darkened field of the polarizing mi- 

 croscope they are luminous and splendidly coloured, the 

 colom's depending upon the thickness of the crystal, and 

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

 the httle inclined plane which fonns its extremity ; but 

 upon oblique portions are seen three or four bands of 

 colour parallel to the edge and offering to the eye a 

 visible scale or measure of the rapid diminution of thick- 

 ness in that part. The observed succession of colours 

 in one experiment was the following : — yellow, browTi, 

 purple, blue, sky-blue, straw-yellow, pink, green, pink, 

 blueish green, pink. Sulphate of copper, with a drop of 

 nitric ether 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 lil:e an assemblage of brilhant rubies, topazes, 

 emeralds, and other gems, each being of a different thick- 

 ness, depolarizing the light in a different degree. If the 

 polarizing eye-piece be now turned a quarter round, the 



