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XVI. 'Description of a single-lens Micrometer, By William Hyde 

 Wollaston, M. D. Sec. R. S, 



Read February 25, 1813. 



Having had occasion to measure some very small wires 

 with a greater degree of accuracy than I was enabled to do by 

 any instrument hitherto made use of for such purposes, I was 

 led to contrive other means that might more effectually answer 

 the end proposed. The instrument to which I had recourse is 

 furnished with a single lens of about ^ of an inch focal length. 

 The aperture of such a lens is necessarily small, so that when 

 it is mounted in a plate of brass, a small perforation can be 

 made by the side of it in the brass as near to its centre as tt 

 of an inch. 



When a lens thus mounted is placed before the eye for the 

 purpose of examining any small object, the pupil is of suffi- 

 cient magnitude for seeing distant objects at the same time 

 through the adjacent perforation, so that the apparent dimen- 

 sions of the magnified image might be compared with a scale 

 of inches, feet, or yards, according to the distance at which it 

 might be convenient to place it. A scale of smaller dimen- 

 sions attached to the instrument will, however, be found pre- 

 ferable on account of the steadiness with which the comparison 

 may be made; and it may be seen with sufficient distinctness by 

 the naked eye, without any effort of nice adaptation, by reason 

 of the smallness of the hole through which it is viewed. 



