132 MICROSCOPIC ILLUSTRATIONS. 



diamond and garnet, are perfectly spherical ; and, owing 

 to the great refractive power of these stones, they have 

 the appearance of black and opaque globules with a 

 small bright spot of light in their centres. The propor- 

 tion between the diameters of the dark annulus and the 

 luminous spot, Sir D. Brewster has ingeniously proposed 

 as a measure of the refractive powers of both the solid 

 and the fluid. He has suggested, also, that these cavities 

 being permanent, are preferable to Dr. Goring's* air 

 bubbles in Canada balsam, for determining the aberra- 

 tions of object-glasses ; but a little consideration, 1 

 think, will suffice to shew that both the one and the 

 other of these tests must give erroneous results for 

 ordinary objects. And for this simple reason, because 

 the light coming from the cavity to the object-glass 

 through a portion of the gem, will be refracted by it, 

 and enter the object-glass in a different state to what 

 it would have done had no such medium intervened 

 between the cavity and the lens. 



In examining the organic remains embedded in flint, 

 the object-glass requires to be corrected by this test, as 

 the rays of light proceeding from the creatures dis- 

 coverable in thin plates of that substance, have to pass 

 through it before they enter the object-glass. The 

 recent discovery of the crystatella f and other infusoria 

 in flint, as also the sporules of ferns, has opened an en- 



* See Micrograpbia, page 107. 

 f Zanthydium furcata of E. 



