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XXIV. — On Micrometers. By Geokge Jackson, Esq. 

 (Read November 24, 1847). 



It is often desirable in microscopical investigations to have a ready 

 means of ascertaining the dimensions of the objects we are examining ; 

 and this want was felt by microscopists at a very early period. 

 Leuwenhoeck, the father of microscopical discovery, selected minute 

 grains of sand as nearly alike as possible ; and, arranging them in a 

 line, counted the number which occupied the space of an inch. By 

 comparing the subjects of his examination with these, he was enabled 

 to form a rough estimate of their bulk. 



This was a rude contrivance, and suited to the infancy of the mi- 

 croscope ; but one on a similar principle, although rather more 

 refined in practice, has been recently proposed by a distinguished 

 German botanist, and an enthusiastic advocate of microscopical 

 research. Schlegel recommends that we should take as our standard 

 the blood-corpuscles of some common animal, and estimate the size 

 of other bodies by comparison with them. 



Leaving the history of micrometry as a subject more curious than 

 useful, we will proceed to the methods at present practised. 



Lines can be ruled with a diamond one thousand to the inch, or 

 even finer, with considerable accuracy ; and the idea would naturally 

 occur to any one that were an object laid on a slip of glass so divided, 

 it could be measured as we measure larger bodies with a rule. This, 

 however, except with a low power, is quite impracticable ; for the fo- 

 cus of a good microscope is so precise, that the plane in which 

 objects can be distinctly defined is almost a mathematical one, which 

 cannot be occupied by two bodies at the same time : and besides, if 

 the object be immersed in a fluid, as is generally the case, the lines 

 become filled with it, and are thus rendered invisible. 



The measuring of the object itself being then impossible, let us turn 

 our attention to its magnified image : and here a variety of methods 

 have suggested themselves to microscopists. 



