optics. 295 



found to give some trouble in understanding and 

 computing the dimensions taken by it; and, there- 

 fore, it is here applied to the stage, or rather to the 

 object itself; which renders the use of it more 

 pleasant. 



The upper part of the microscope, which con- 

 tains the glass, has a fine wire in its focus, and to it 

 any part of the image maybe applied; or the wire 

 may be applied to any part of the image, by a proper 

 construction of the upper part of the microscope 

 for that purpose. The object being then placed in 

 a proper manner on the stage, the screw is to be 

 turned till the image of the object has passed its 

 whole length or breadth under the wire, and then 

 its dimensions will be known. Thus, for example: 



The number of threads on the screw in one inch 

 is fifty, and the number of divisions on the circular 

 plate is twenty. Therefore one thread, or one 

 turn of the screw, measures one-fiftieth part of an 

 inch: and one division of the plate is one-twentieth 

 of one-fiftieth, that is, one-thousandth part of an 

 inch. So that such a micrometer will very exactly 

 measure any small object, or its smaller parts to 

 the thousandth part of an inch. 



Suppose the subject were a mite, and it were re- 

 quired to measure the length of it; place it in a 

 slider, and that slider on a stage, in such a manner 

 that the mite may move lengthwise in the direction 

 of a screw; then move or set the wire at right 

 angles to it, and so as touch the image of the mite 

 at one end, very exactly. This done, turn the 

 wire till the image has passed its whole length 

 under the wire ; and having counted the turns, you 

 will find them four, and fourteen divisions of 

 another; the four turns are 4% or T §So, and the 

 fourteen divisions are T-iSo; so that the whole length 



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