MICROSCOPIC OBJECTS. 51 



ingly, Mr. P. also assigning the reasons that induced him 

 to make choice of it. Dr. L. with great skill and per- 

 severance subsequently made several preparations, in 

 which he most beautifully dissected out the spiral fibre 

 with its inclosing sheath. This object requires a mag- 

 nifying power of from 300 to 500 diameters, with great 

 defining power, considerable penetration, and a pure 

 illumination. 



Oysters, young. — During the spawning season, if an 

 oyster be cut open, a thick opaque fluid may sometimes 

 be noticed. If a little of this is taken up on the point 

 of a scalpel or penknife, and rubbed on a hard substance, 

 and it has a gritty feel, it is almost certain to be the ova. 

 To examine these under the microscope, place a drop of 

 this thick fluid on a slip of glass, and cover it with 

 another thin one; and it is then ready for observation. 

 These ova are of the same form as the parent, but more 

 convex. The shell is perfect, and the muscle which holds 

 the two valves together may be distinctly seen through 

 the transparent shell. They are often not more than the 

 ^^^th of an inch in diameter. They may be mounted as 

 permanent objects. — Magnifying power 200 diameters. 



Pearl, Nacre, or Mother -of -Pearl. — ^To exhibit the true 

 formations of this structure it should be cut in two 

 directions; that is to say, two specimens must be pro- 

 cured, the one a thin section cut parallel to the shell, and 

 the other vertical to it. The former exhibits the mode 

 of the cropping out of the layers ; the edges and folds 

 produce the beautiful colours by which this substance is 



