54 CONSTRUCTION OF THE MICROSCOPE. 



out, and the number of divisions on the eye-piece micro- 

 meter equivalent to an inch on the stage ; and any 

 measurements afterwards taken with that micrometer and 

 object-glass may, by a short process of mental arithmetic, 

 be reduced to the decimal parts of an inch, if not actually 

 observed in them. 



" In ascertaining the value of the micrometer with a 

 deep object-glass, the hundredth of an inch on the stage 

 will occupy too much of the field ; the two-hundredth or 

 five-hundredth should then be used., and the number of 

 divisions corresponding to that quantity be multiplied by 

 two hundred or five hundred, as the case may be. 



" The micrometer should not be fitted into too deep an 

 eye-piece, for it is essential to preserve clear definition. 

 TH middle eye-piece is for most purposes the best, pro 

 vided the object-glass be of the first quality ; otherwise, 

 t^se the eye-piece of lowest power. The lens above the 

 micrometer should not be of shorter focus than three- 

 quarters of an inch, even with the best object-glasses; and 

 the slit cut in the tube can be closed at any time by a 

 small sliding bar, as at Z, fig. 35." 



We subjoin the following comparative micrometrical 

 measures given by Dr. Hannover, as a reference-table. 



The wonderful tracing on glass executed by M. Nobert, 

 of Earth, in Prussia, deserves attention. The plan adopted 

 by him is, to trace on glass ten separate bands at equal 

 distances from each other, each band being composed of 

 parallel lines of some fraction of a Prussian inch apart ; in 

 some they are l-1000th, and in others only 1 -4000th of a 

 Prussian inch separated. The distance of these parallel 

 lines forms part of a geometric series : 



