326 Mr. H. F. Talbot's Experiments on Light. 



often observed in a state of rapid growth, increasing in all 

 their dimensions, while they accurately retain the same geo- 

 metrical figure. With the polarizing microscope they are 

 seen to change their tint as they increase in size, and thus, 

 for instance, it often happens that a crystal which possessed 

 a red colour at the moment of the first observation, if viewed 

 again after the lapse of a single minute, may have turned to 

 green; and again, after the lapse of another minute, it will, 

 perhaps, have become blue. 



I have said enough to show that I regard the polarizing 

 microscope as an instrument of great promise, and although 

 I have not had leisure to undertake any very extensive series 

 of experiments with it, yet I will add two instances by way of 

 illustration of the manner in which it develops the structure 

 of transparent bodies, and I will choose for my examples the 

 sulphate of potash and the acetate of copper, in both of which 

 I am able to confirm the phenomena announced by preceding 

 observers. 



Sulphate of Potash. — In the yearl819, Sir David Brewster 

 discovered that the bipyramidal sulphate of potash, which had 

 been always considered as a simple crystal, was, on the con- 

 trary, a compound of three others, very curiously jointed to- 

 gether, in such a way as to produce a six-sided pyramid. 



This he ascertained by making a section of the crystal and 

 viewing a plate of it by polarized light. (See Edinburgh Phi- 

 losophical Journal, vol. i. p. 6.) 



But by proper management I find the sulphate of potash 

 may be made to crystallize in thin hexagonal plates, and thus 

 their structure may be examined without the trouble of cutting 

 them. When a perfect hexagon of this kind is viewed in the 

 polarizing microscope, it is seen to be divided into six co- 

 loured triangles, bounded by the six lines which go from the 

 centre of the hexagon to its angles. There are, however, only 

 three different colours, because each triangle has always the 

 same colour with the one opposite to it. It is worth remark 

 that the six boundary lines are visible even by common light, 

 resembling small veins or cracks in the crystal. 



Acetate of Copper. — In this salt Sir David Brewster detected 

 the beautiful property of dichroism. (See Phil. Trans, foi- 

 ls 19, or Brewster's Optics, p. 252.) 



By cutting a plate of it from a large crystal, and examining 

 it by polarized light, he found that it changed its tint from 

 blue to green according to the position in which the crystal 

 was held. Now this curious observation receives the most 

 ample confirmation from the polarizing microscope. I found 

 the following a convenient method of performing the experi- 



