112 MICROSCOPIC ILLUSTRATIONS. 



curious action on natural and artificial bodies, eliciting*, 

 as it does, a brilliancy of colours past all conception, lias 

 created so deep an interest in the minds not only of men 

 of science, but of all who have ever witnessed it, that a 

 treatise upon an instrument so intimately connected 

 with it as the microscope is, would, I am sure, be deemed 

 altogether incomplete, if it did not embrace some par- 

 ticulars respecting 1 this great phenomenon. The trans- 

 mission of light through two thin plates of crystal is an 

 operation which can be readily enough understood ; but 

 that the mere turning of one of them a quarter round 

 should occasion a total stoppage of light, although the 

 thickness of the plates and their inclination to each other 

 remain the same, is so contrary to the general laws of 

 optics and to our daily experience, that we should be at 

 once disposed to question the fact, if we were not prac- 

 tically convinced of its truth. Again, supposing the 

 plates to be in the position last named, by which, as we 

 have said, the light is intercepted in its passage through 

 them, and it was suggested to us to interpose between 

 them an additional plate of a crystal, the natural conclu- 

 sion would be, that this arrangement would operate to 

 occasion a greater exclusion, if possible, than did the 

 other, which, under ordinary circumstances, would doubt- 

 less be the case. Having done this, however, we find, 

 to our great astonishment, that the light is now freely 

 transmitted, and often accompanied with the most in- 

 tense and splendid colouring. To arrange plates of 

 crystals in chaste and pretty designs, so as to make these 



