MICROMETER. 217 



ON THE MEASUREMENT OF MICROSCOPIC DEJECTS. 



First. — With the Stage Micrometer. — To effect this it is 

 necessary to be provided with a strip of glass, mother-of- 

 pearl, or ivory, on which lines are accurately ruled at a 

 certain fraction of an inch apart, >say from the -^l^ to the 

 TsViT of an inch, as was done by the late Mr. Coventry and 

 Sir John Barton. The most convenient form of micrometer, 

 for all purposes, wUl be one divided into hundreds, and one of 

 these divisions into ten parts, or thousands. Those of glass 

 are the best for transparent objects, and the mother-of-pearl, 

 or ivory, for opaques; and, to make the lines more evident, 

 finely levigated blacklead should be rubbed in to fill them up. 

 The object to be measured is to be laid on the glass, or 

 mother-of-pearl, and both it and the lines must be viewed at 

 one and the same time, with the lowest power that can con- 

 veniently be used ; the number of the divisions occupied by 

 the object will give the measurement required. Example : — 

 Thus, suppose an object occupied ten of the large divisions, 

 its linear measurement would then be -^^, which if reduced 

 to its lowest terms, would be the -^^ of an inch ; if fifty, then 

 it would measure half an inch, if fifteen divisions, then it 

 would be -jJ/g, or nearly a of an inch. It follows, therefore, 

 that an object to be measured in this way must be very thin 

 and the power low, in order that it and the lines may be in 

 focus at the same time. This micrometer answers better for 

 the simple microscope than for the compound achromatic 

 instrument, as in the latter the focus even of a low power 

 object-glass is so exact, that but very few objects are thin 

 enough to be seen at the same time as the lines. Most of the 

 objects requiring measurement are those which must of 

 necessity be examined in fluid; this will render the lines 

 on the glass micrometer all but invisible, unless they are 

 very boldly ruled and filled up with some opaque matter. 

 . Mr. Pritchard supplies with his microscopes animalcule cages, 

 on the upper surface of the bottom glass of which, lines arc 



