DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 101 



we cannot enjoy the sight of their motions and habits as 

 with living specimens, yet are we in a measure recom- 

 pensed for it, by the advantage of having them always 

 at hand. 



The object-glasses, C, are best adapted for investi- 

 gating objects of the most minute and delicate descrip- 

 tion ; such, for instance, as the markings on the scales of 

 the podura ; the forms and structure of minute fungi* ; 

 the globules of human blood, &c. 



A variation of magnifying power is attainable also by the 

 body of the instrument being composed of two or more 

 tubes (see fig. 17,) the one sliding within the other, so as 

 to admit of an alteration of distance between the eye-piece 

 and object-glass : the power, in this case, will vary in the 

 direct proportion of the interval between the eye-piece 

 and object-glass. I am fully aware that this alteration 

 in the conjugate foci of the object-glass will affect its 

 corrections ; still, in some cases, the advantages to be 

 derived from it will counterbalance this objection. 



Having now given a description of the instrument in 

 its simplest mounting, together with examples for select- 

 ing its object glasses and eye- pieces, I shall proceed to 

 consider, separately, the several additions which can be 

 made to it, inasmuch as each one of them can be em- 



* See Rev. M. J. Berkeley's admirable paper on Fungi, Annals Rat. 

 Hist. vol. i. p. 82. 



