VISUAL WORK WITH THE MICROSCOPE, 491 



5. On the power of the objective. The lower the power of the 



objective the smaller, with the same condenser, will be 

 the image of the collecting lens. 



6. On the exact centring of light, collecting lens and condenser. 



A 2. The intensity of the illamination which, however, is of 

 more importance in photomicrography depends on several factors, 

 but here I shall only refer to three of them : 



1. The source of light (lamp or electric light, etc.). 



2. With the kerosene lamp, whether we use the flat or the 



edge of the flame. 



3. The use of coloured screens. 



B. T/ie Filling of the Back Combination of the Objective ivith 

 Light. 



It has been laid down by Abbe that the resolving power of 

 an objective depends on its IsT.A. If this be admitted, one can 

 understand how important it is that the result of the illumina- 

 tion should be such as to enable us to avail ourselves of the full 

 N.A. Not that it is always possible to employ usefully the full 

 aperture. 



The filling of the objective with light depends on the following 

 conditions : 



1. That the substage condenser should be capable of focusing on 



the plane of the object a sharp image of the source of 

 light, and should possess an ajyla^iatic aperture at least as 

 large as that of the objective in use. 



2. That the sharp image of the illuminant, focused by the 



collecting lens on the mirror, should be large enough 

 to fill the opening of the iris necessary to give us the 

 full N.A. of the objective. (And here it may be re- 

 marked, that the farther the lamp and collecting lens 

 are from the microscope, the larger will be the projected 

 image of the edge of the flame.) 



3. That the lamp, collector and condenser should be centred, 



as before described. 



4. Condensers. 



It is hardly necessary to say that without a condenser 

 critical work, at any rate with high powers, is impossible. Even 



JouRN. Q. M. C, Series II. No. 71. 33 



