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1900] MICROSCOPICAL JOURNAlX^^^TB V. 



Before leaving the subject of illumination we will deal 

 with the bull's-eye condenser. In one of our earlier papers 

 (Science-Gossip, Vol. VI., N.S., page 215) we have alluded 

 to the enormous spherical and chromatic aberrations of 

 this lens, and these render it unsuitable for really criti- 

 cal work on account of its bad definition. For this reason 

 we do not advocate its general use, save for opaque il- 

 lumination. Should it be necessary, however, to fill the 

 field with light by its means (see page 215) we would ask 

 our readers to bear its optical properties in mind and to 

 remember that to obtain parallel light the condenser must 

 be placed close to the flame of the lamp, and with its flat 

 side against the flame. The bull's-eye must, of course, 

 also be placed both centrally and at right-angles with the 

 direction of the light. It is an assistance to beginners if 

 they do their focussing, both with bull'seye,and even with 

 condenser, upon a sheet of note-paper placed in the re- 

 quisite position. If the bull's eye be used, it must be prop- 

 erly and carefully adjusted, or it will only interfere with 

 the proper focussing of the condenser. 



To pass now to the focussing of the object itself. This 

 needs but little explanation, but it may again be advisa- 

 ble to point out (see page 157) that the so-called focal 

 length of the objective does not in any way represent its 

 distance from the cover glass of the object. In fact, with 

 increase of aperture, the objectives have got closer and 

 closer to the object. When using high powers it is a help 

 and sometimes a preventive of damage, if the aperture of 

 the stage, as is customary in English stands, is made suf- 

 ficiently large to admit of the insertion of the finger un- 

 derneath the slide so as to slightly lift or tilt its fore-edge. 

 A high power can then be safely brought down upon the 

 object, and the approximate distance being found, the fin- 

 ger can be removed, and the objective then brought gent- 

 ly to its ultimate focus. 



We do not think it advisable here to explain the use of 



