MICEOMETER. 20S 



CHAPTER IV. 



MICROMETER. 



One of the most valuable adjuncts to a good microscope is 

 an instrument termed the micrometer, which, as its name 

 implies, is for the purpose of ascertaining the measurement of 

 small objects; for not only is it highly desirable in micro- 

 scopical investigations to have a means of estimating the exact 

 size of any object under examination, but in the present 

 advanced state of science, accurate measurements oftentimes 

 form the most valuable points of distinction, and from the ear- 

 liest period of microscopical science, the want of some common 

 standard for comparison has been greatly felt. Leeuwenhoek 

 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 examina- 

 tion with these, he was enabled to form a rough estimate of 

 their bulk. More modern microscopists have employed for 

 the same purpose the sporules of the Lycoperdon bovista, or 

 puff-ball. These, which are said to be so smaU as the g^^TyTj 

 part of an inch, will only answer for the most minute structures, 

 but for those that are larger, the sporules of the Lycopodium 

 may be used, their mean diameter being about -g-J^ of an inch. 

 Dr. Jurin* introduced into the field of view small pieces of 

 silver or brass wire, the diameter of which he had previously 

 ascertained by coiling it round a cylinder, and observing how 

 many breadths of the wire were contained in a given number of 

 inches, or parts of an inch. Other substances, whose dimen- 

 sions have previously been made known, such as hair, sills:, 

 and human blood, have all been suggested as applicable to the 

 same purpose. As long ago as 1750,t Martin Folkes, then 



* Physico Mathimatical Dissertations, p. 45. 

 f Baker — On the Microscope, vol. ii., p. 426. 

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