MICROSCOPIC OBJECTS. 37 



An intense light is to be directed through it, and the instrument 

 adjusted to the object in the usual way, which will present the same 

 appearance as without the tube of prisms. If, however, the second 

 tube be placed near the eye, and close to the eye-glass, the object will 

 appear of the most brilliant colours, if affected by polarized light. 



When an object under examination exhibits the colours by 

 depolarizing the light, if the field of view appear luminous, as in 

 viewing transparent objects by common light, cause the eye-piece with 

 its prism-tube to revolve ; and in certain positions, the field of view will 

 appear black the objects assuming at the same time the complimen- 

 tary colours, and appearing like brilliant gems lying upon black 

 velvet. Many crystals exhibit these polarized tints very intensely. 

 The following, crystallized on a slip of glass, are remarkably interesting, 

 both as regards the elegance of their form and the splendour of their 

 colours. Chlorate of potash, oxalic acid, prussiate of lime, nitrate of 

 potash, and acetate of copper. The great advantage of employing the 

 microscope in viewing the polarized tints of bodies is, that very small 

 specimens will answer equally well with larger and more expensive 

 ones in the ordinary way ; and that they do not require any trouble- 

 some processes to cut them of different thicknesses for obtaining the 

 different tints, this being accomplished in the process of the chrys- 

 tallization. 



26. Mr. Talbot has directed his attention to this subject ; and as 

 the experiments he has published are exceedingly interesting, and may 

 readily be repeated, I have subjoined some of them : 



Sulphate of copper, which is of a fine blue colour when viewed in 

 considerable thicknesses, is white and transparent when it is extremely 

 thin, and its chrystals can be procured so small as to be quite destitute 

 of perceptible coloration. A drop of its solution was placed upon a 

 warm piece of glass, 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 end of a chisel. Seen 

 by common light, these crystals offer nothing peculiar ; but on the 

 darkened field of the polarizing microscope, they are luminous and 

 splendidly coloured, the colour depending upon the thickness of the 



