MICROMETER. 213 



it consists of two tubes, sliding one within the other; the 

 external one has the divided glass screwed into its lower end, 

 and the internal carries the two lenses, as shown in fig. 138. 

 When this figure is compared with fig. 101, it will be seen 

 that the field-lens is reversed ; that is, its convex surface is 

 towards the eye. The divided glasses are shown by fig. 139 ; 



the lines ruled on them 

 may vary from the -^ to 

 the ^Jjf of an inch apart; 

 they are set in brass cells, 

 by which they may be 

 ■p- jgg screwed into the lower end 



of the outer tube, so as to 

 be in the focus of the eye-glass. The value of the squares, 

 yf'ith the different eye-pieces, is obtained by a stage micro- 

 meter. Mr. Koss now employs only one divided glass, and 

 the values of the squares, with the diflFerent object-glasses, 

 having been determined, are set down in a tabular form. 

 Mr. Powell and Mr. Smith, following Mr. TuUey, place the 

 micrometer in the negative eye-piece in the situation of the 

 stop. Each of these plans has its peculiar advantages. The 

 positive eye-piece gives the best view of the micrometer, the 

 negative of the object. The former is quite free from distor- 

 tion, even to the edges of the field, but the object is slightly 

 coloured. The latter is free from colour, but is slightly dis- 

 torted at the edges ; in the centre of the field, however, to 

 the extent of half its diameter, there is no perceptible distor- 

 tion, and the clearness of the definition gives a precision to 

 the measurement which is very satisfactory. The plan now 

 generally adopted is that which was first recommended by 

 Mr. George Jackson, in a paper read before the Microscopical 

 Society, in 1840; since that time, he has improved upon the 

 method of mounting the divided glass, and has furnished to 

 the society a more lengthened communication, from which, by 

 his permission, I have taken my description. 



Short bold lines are ruled on a piece of glass ; and, to 

 facilitate counting, the fifth is drawn longer, and the tenth 

 still longer, as in the common rule. Very finely levigated 



