Microscopical Essays. ^ 



2. A piece of glafs, Fig. 2, fixed in a Draft or ivory flic'er ; on 

 the diameter of this are drawn two parallel lines, about three- 

 tenths of an inch long • each tenth being divided, one into three, 

 the fecond into four, the third into five parts. To ufe this, place 

 the glafs, Fig. 2, on the middle of thefiage, and the rule, Fig. 4, 

 on one fide, but parallel to it ; then look into the microfcope 

 with one eye, keeping the other open, and obferve how many 

 parts one-tenth of a line in the mkrofcope takes in upon the 

 parts of the rule feen by the naked eye. For inftance, fuppofe 

 with a fourth magnifier, that one-tenth of an inch magnified 

 anfwers in length to forty tenths or parts on the rule, when feen 

 by the naked eye, then this magnifier increafes the diameter of 

 the object forty times. 



This mode of actual admeasurement is, without doubt, the 

 mod fnnple that can be ufed ; by it we comprehend, as it were, 

 atone glance, the different effecls of combined, glafles ; it faves 

 the trouble, and avoids the obfcurity that attends the ufual modes 

 of calculation ; but many per fons find it exceedingly difficult to 

 adopt this method, becaufe they have not been accufiomed to 

 obferve with both eyes at once, We (hall therefore proceed to 

 defcribe another method, which has not this inconvenience. 



Of the Needle Micrometer. 



'Fig. 8, Plate II. A, reprefents this micrometer. The firft of 

 <£his kind was made by my father, and was defcribed by him in 

 his Micrograph ia Illuftrata. It confifts of a fcrew, which has 

 ffiy threads to an inch ; this fcrew carries an index, which points 

 to the divifions on a circular plate, which is fixed at right angles 



II 2 to 



