144 HOW TO SEE WITH THE MICROSCOPE. 



diatom, placing- the same in position under the stage 

 of the histological, I got handsome resolutions with 

 the mirror, at say 80 from axis, and also when the 

 swinging-bar was posed so that more than half the 

 mirror was above the sta^e. 



C5 



Tested for working distance. I found the same to bo 

 .015, equal, say, to one-sixtieth of an inch. 



There were very many other " rounds" to this fight, 

 which are purposely omitted as having no bearing on 

 the subject in hand. The reader has before him the 

 work of the objective by central, centrally disposed, 

 tolerably oblique and decidedly oblique illumination. 



Now, thus having the work of the objective before 

 us, it is palpably evident tliat it has all desirable work- 

 ing distance. Surely no one but a bungler would at- 

 tempt to cover a nice mount with glass over one one- 

 hundredth of an inch in thickness, while, with such a 

 cover, the one-tenth will have plenty of room and to 

 spare; and if we compare the glass named with the 

 usual run of immersion tenths, it will be obvious that 

 there is no loss of working distance nor any sacrifice 

 thereof in any way, shape, or manner. 



OBJECTIVES OF LOWER BALSAM ANGLE. 



These, when compared with the glasses we have just 

 had under consideration, will be found to have greater 

 working distance, as has been before stated; and with 

 this increase of distance there will be greater focal 

 depth the so-called " penetration " of the books. 

 Now, in the preliminary investigation of many objects, 



