182 MICROSCOPIC ILLUSTRATIONS. 



the longest eye-pieces, having the largest glasses in them, 

 are also the lowest powers. Now the total power of the 

 body or optical part of the engiscope, is the result of that 

 of its object-glass multiplied by or combined with that 

 of the eye-piece ; therefore the highest power will be 

 given by the smallest object-glass used with the shortest 

 eye-piece, and the lowest by the largest object-glass with 

 the longest eye-piece. The eye- pieces mentioned above 

 are all of the inverting kind, like those belonging to 

 astronomical telescopes ; there is, however, a pair of 

 erecting glasses, A, to screw into the bottom of the pull- 

 out tube, which, with the former, make erecting eye-pieces ; 

 these are used for dissecting, &c., and also for giving 

 very low powers when wanted below the scale of the 

 weakest inverting ones. The use of all these I shall 

 shew in their proper places. Each object-glass combines 

 with each eye-piece, whatever their number may be, 

 according to the fancy of the observer, but the power 

 which results from the combination can only be known 

 by trial, and should be given by the maker of the instru- 

 ment, in a written paper. 



The object-glasses are frequently made to combine 

 with each other. An increase of power is obtained by 

 the elongation of the body, effected by drawing out the 

 tube, 'c : whenever the length of the body is doubled, for 

 example, the power is also doubled (cateris paribus). 



In winter it will be proper to cause the instrument to 

 be slightly warmed at the fire before it is used, until it is 

 of about the same warmth with the human body, other- 



