Microscopical Essays. 61 



affiftance towards afcertaining an accurate idea of it's real fize ; 

 for our ideas of bulk being formed by the eomparifon of one ob- 

 ject with another, we can only judge of that of any particular 

 body, by comparing it with another whofe fize is known : the 

 fame thing is neeeflary, in order to form an eftimate by the 

 microfcope ; therefore, to afcertain the real meafure of the ob- . 

 jea, we muff make the point of the needle pafs over the image of 

 a known part of an inch placed on the ftage, and write down the 

 revolutions made by the fcrew, while the needle palled over the 

 image of this known meafure ; by which means, we afcertain the 

 number of revolutions on the fcrew, which are adequate to a real 

 and known meafure on the ftage. As it requires an attentive eye 

 to watch the motion of the needle point, as it pafles over the 

 image of a known part of an inch on the ftage, we ought not to 

 truft to one {ingle measurement of the image, but ought to repeat 

 it at leaft fix times ; then add the fix meafures thus obtained to- 

 gether, and divide their fum by fix, or the number of trials ; the 

 quotient will be the mean of all the trials. This refult is to be 

 placed in a column of a table, next to that which contains the 

 number of the magnifiers. 



By the affiftance of the feaoral fcale, we obtain with eafe a 

 • Small part of an inch. This fcale is {hewn at Fig. 5, 6, 7, Plate 

 II. A, in which, the two lines ca cb, with the fide ab, form an 

 ifofceles triangle ; each of the fides is two inches long, and the 

 bafe one-tenth of an inch. The longer fides may be of any given 

 length, and the bafe Ml only of one-tenth of an inch. The 

 longer lines may be confidered as the line of lines upon a kBor 

 opened to one-tenth of an inch. Hence, whatever number of 

 equal parts ca cb are divided into, their tranfverfe meafure will. 



