MICROSCOPIC OBJECTS. 185 



the object-glass you intend to use with this eye-piece, 

 screwit in to the body, and fit in the eye-piece in the same 

 manner as if it was of the ordinary kind ; select your 

 object, and bring it to a proper focus. 



It is now necessary to examine whether the divisions 

 on the Micrometer are distinct ; if not, they are adjusted 

 to distinct vision in the following manner : — Unscrew 

 the diaphragm cap on the top of the eye-piece, and give 

 the eye-lens one or two turns until the micrometer 

 is brought into focus. The ceU of the eye-lens has a 

 very long screw to permit of this adjustment. 



Having made your adjustment, the last thing to be 

 done is to find the value of each division on the micro- 

 meter. Place on your stage instead of the object a mi- 

 crometer divided into -y^^ths of an inch or any other 

 suitable division, w^hich will depend upon the power of 

 the object-glass. Let us suppose in this case the divi- 

 sions of the micrometer on the stage to be y^ oth of an 

 inch, and that when looking through the instrument each 

 of such divisions appears to cover 10 divisions of the mi- 

 crometer in the eye-piece ; it follows then, if an object be 

 substituted for the micrometer on the stage, that each 

 division which the object covers of the micrometer in the 

 eye-piece will be equal to yoVo^^ ^^ ^^ inch. Suppose, 

 as an example, it is desirable to measure the diameter of 

 a hair, the apparent diameter of which in the microscope 

 is five divisions of the pearl or glass micrometer in the eye- 

 piece ; it follows that the real diameter of the hair is equal 

 to x^%o^h of an inch, that is ^Joth of an inch. If, how- 



