MICROMETER. 2 1 1 



isosceles triangle, each of whose sides was two inches long 

 and the base one- tenth of an inch ; one side was divided into 

 ten parts, and the transverse measure was such a part of the 

 tenth of an Inch as was expressed by the divisions, so that 

 the transverse measure of the first division would be the 

 tenth part of a tenth of an inch, or in other words, the -yvs- 

 If the divisions were twenty in number, then the value of the 

 same measure would be 2^^ of an inch. Adams also con- 

 trived a micrometer for placing on the stage ; it consisted of 

 a few small silver wires, in the form of a lattice, the distance 

 of one from the other being exactly equal to that of the 

 diameter of the wire itself; the lattice was placed between 

 two pieces of mica, and the object was placed upon the mica, 

 and both it and the lattice were magnified equally. Various 

 other contrivances have been had recourse to for effecting the 

 same object, but they may all be classed under two heads : — 

 first, those micrometers that are applied to the magnified 

 image of an object, and second, those that are magnified 

 at the same time as the object itseF. It would, however, be 

 useless to enter further in detail into the subject, as only a few 

 of the old forms are now adopted, these being the stage micro- 

 meter, consisting of a number of hues accurately ruled on 

 glass, metal, ivory, or mother-of-pearl, after the plan of those of 

 the late Mr. Coventry, and the cobweb micrometer eye-piece 

 of Kamsden: the divided object-glass micrometer, and many 

 others equally good, being now but rarely employed with 

 the achromatic compound microscope. The micrometers in 

 general use at the present day are of three kinds, and may be 

 designated as follows: — first, the stage micrometer, second, 

 the micrometer eye-piece, and third, the cobweb or wire 

 micrometer. The first, or stage micrometer, is placed in 

 immediate contact with the object, and both it and the object 

 are magnified together; whereas the last two are applied to 

 the magnified image of the object, which in practice has been 

 found the most available plan. 



Staffe Micrometer. — This instrument, invented by the late 

 Mr. Coventry, consists of a piece or slip of glass, metal, ivory, 

 or mother-of-pearl, ruled with fine lines, by means of a diamond 

 14* 



