SPECIAL APPARATUS 21 



It is well to remember that with this method one 

 cannot afford to waste much time in manipulating the 

 adjustments of the microscope. The cells, under some 

 conditions, die quickly, and we therefore have to search 

 the specimen very rapidly before "achromasia" occurs, 

 when all the cells vanish, as will be presently described. 

 It is better, therefore, to have everything ready before 

 the specimen is prepared. 



The microscope should be fitted with a nose-piece, 

 so that the objective can be changed quickly. When 

 using the immersion lens, great care must be exercised 

 in placing the drop of cedar oil on to the cover-glass, 

 for the cells and jelly-films are easily destroyed if it is 

 accidentally touched with the solid oiler. There is 

 neither time nor necessity to reverse the mirror from 

 concave to plane when the objective is being changed 

 from a dry to an immersion one. When searching 

 through the specimens of living cells, rapidity of 

 focusing will be found to be of more value than too 

 much attention to accurate microscopy, which is 

 difficult, if not impossible, to adhere to with this 

 method. The focusing of the substage condenser on 

 the specimen cannot be very accurate. Most micro- 

 scopes are adjusted for slides of a certain thickness, 

 but we have to place a comparatively thick film of 

 jelly on top of the slide, and hence the objective 

 is always farther away from the condenser than" it 

 ought to be. 



The photomicrographic apparatus (figs. 2-5) in- 

 vented for this method has been designed so that a 

 photograph can be obtained quickly of any field in 



