52 MICROSCOPIC OBJECTS. 



distinguished, the reflected ray from one groove inter- 

 fering with that from the adjoining one. The vertical sec- 

 tion shows the thickness and superposition of the several 

 layers. To exhibit the lines which occasion the colour, 

 a power of 1 70 diameters is necessary. The layers may 

 be seen with a power of 60. In consequence of the 

 reflection from the edges of the layer, nacre is more 

 translucent in one direction than in another. 



Pearl. — This form of secretion of nacre, which is so 

 much prized for ornamental purposes, is considered as 

 resulting from a disease, or rather that small fragments 

 of a foreign hard substance having obtruded themselves 

 within the shells of the mollusk, the creature covers 

 them with layers of the nacre (commonly called mother- 

 of-pearl) that lines the interior of the sheU. If a pearl 

 be cut into very thin sections, the concentric layers are 

 readily distinguished under the microscope (power 100 dia- 

 meters). The central mass varies in almost every pearl. 



Proboscis of a Bee. — Few objects when mounted in 

 balsam yield to this in variety of form and richness of 

 colour. Swammerdam has given a foho drawing of this 

 object in his work entitled ' The Book of Nature.' To 

 furnish a fuU description of this organ would occupy many 

 pages ; the reader is therefore referred to the object 

 itself. A perfect specimen well prepared is of much 

 value ; but good ones mounted in balsam may be had for 

 eighteenpence. — Magni^dng power, for general view 30 

 diameters ; for details, 175 diameters. 



Jaws of Mollusca. — The manducatory apparatus of 



