MICROSCOPIC OBJECTS. 59 



fish of the last order ; hence a fish- scale is formed of 

 layers, and as each succeeding layer is larger than the 

 previous one, when a cycloid scale is viewed through the 

 microscope it exhibits a series of strise of the form of 

 the scale. This form of the scale varies in different 

 genera of fish ; hence they afford good specific charac- 

 ters. Along each side of a fish a straight line or a curved 

 line may often be observed: this is called the lateral line, 

 and is produced by a row of scales furnished with a duct, 

 or tube, from which exudes an oil-hke fluid for lubricating 

 the surface of the fish. Among the various kinds of 

 fish-scales selected for microscopic objects, those of the 

 eel are much prized, as it was formerly considered that 

 that fish had no scales. 



Some scales when viewed by polarised fight have a 

 briUiant effect. Tliey may be mounted in Canada balsam. 

 A magnifying power of 30 diameters is generally suffi- 

 cient for this class of objects. 



FossU scales of fish are often abundant in flint- stones. 

 Those from the gravel- drifts at Gillingham in Surrey, and 

 the flint-nodules in the chalk between Gravesend and 

 Rochester, contain them. To obtam and prepare these 

 specimens for the microscope, select some large nodules 

 of flints and break them up into thin pieces. After each 

 fi'acture, examine the surfaces with a hand magnifier. 

 It is barely possible to reduce a dozen flints from the 

 above locahties without finding some specimens of fish- 

 scales. They are best examined as opaque objects under 

 a very low power. • 



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