218 USE OF THE MICROSCOPE. 



ruled from the ^J^ to the ^|^ of an inch apart; these can 

 hardly be seen when the fluid containing animalcules is 

 present, although they are very coarsely ruled, and, with 

 objects of any thickness, all measurements are incorrect, in 

 consquence of the rays of light from the object and the 

 micrometer not being given off at the same angle ; so that 

 the object is referred to a point of the micrometer larger than 

 it really is. 



Second. — By the Micrometer Eye-piece. — In the description 

 of this instrument, given previously in pp. 212-3, it was stated 

 to consist of an eye-piece in the focus of which a piece of 

 glass, having short bold lines ruled upon its upper surface, 

 was placed generally -^^ of an inch apart, with every fifth 

 longer, and every tenth longer still, to facilitate coimting; 

 but it is not necessary that the number of lines in an inch be 

 known, as long as they are equidistant ; and let us take, in 

 the first place, the negative eye-piece, as supplied with Mr. 

 Powell or Mr. Smith's microscopes. 



To find the Value of the Lines in the Negative Eye-piece 

 Micrometer. — The micrometer set in its brass frame, as seen 

 in fig. 140, is passed so far through the slits in the eye-piece, 

 as that the lines may be seen to occupy the centre of the 

 field of view, and to be in the focus of the eye-glass. The eye- 

 piece having been placed in the end of the compound body, 

 as far as it will go, the next step is to determine the value of 

 the divisions of this eye-piece micrometer with each of the 

 object-glasses. This is done by laying on the stage of the 

 microscope a glass micrometer, divided, say into y-J-g- and Tinrir 

 of an inch, and the lowest object-glass being screwed to the 

 compound body, the lines on the stage micrometer are to be 

 brought into focus; and either the eye-piece or the stage 

 micrometer having been so turned as to bring the lines in 

 both micrometers parallel, we must then observe how far the 

 lines in each coincide, whether every third, fourth, fifth, and 

 so on ; and, for the sake of simplicity, let us suppose the one- 

 inch object-glass to be employed, and it having been noticed 

 that every division of a hundredth of an inch in the stage 

 micrometer coincides with ten in that of the eye-piece, it 



