64 MICROSCOPIC OBJECTS, 



i 



wire) "consists of a straight and strong line, upon which 

 is attached a white and curved Hne, and the whole is sur- 

 rounded by a broad blueish band. There can be no doubt 

 that this blue band consists of lines produced by the 

 spider, and woven into the delicate tissue." See Brew- 

 ster's Journal, new series, vol. vi., p. 64. 



Spines. — Thin transverse sections of the spines of some 

 MammaUa, as the porcupiae, hedgehog, &c., are very 

 beautiful transparent objects for the microscope. The 

 hedgehog spine requires a power of 100 to 150 diameters, 

 while the quUl of the porcupine requires only 30 to 60 

 diameters. The spines of certain Echini, when cut into 

 very thin sections so as to leave only one layer of ceUs, 

 are often exquisitely rich in colour. Dr. Carpenter 

 exhibited some specimens of this kind, very admirably 

 cut, at the Southampton Meeting of the British Associa- 

 tion in 1846. 



Teeth. — The formation of the teeth has of late years 

 been minutely investigated on the Continent by Professors 

 Purkinje and Retzius, and in this country by Professor 

 Owen and others. The preparation of thin sections of 

 teeth being now brought to great perfection, has facilitated 

 the researches of the naturalist, and has thus enabled 

 him, with the aid of the achromatic microscope, to 

 ascertain aU the minutiae relating to their structure. 



A tooth consists of three distinct structures, the relative 

 proportions and arrangement of which constitute the chief 

 differences in the teeth of various animals. 



1. Enamel. — ^This is crystallized phosphate of lime 



