Ch. II] HARK GROUND ILLUMINATION WITH HIGH POWERS 71 



diaphragm inserted or removed. An alternative would be to use the 

 objective for all work with the diaphragm in place, but the reduced 

 aperture would make the resolution of fine details correspondingly 

 less effective. 



The principles involved in high power, dark-ground illumination 

 and for the low-power work are the same, but for the high power 

 work the light for the object must be more oblique 

 or some of it would be at an angle which the large 

 aperture of the objective would admit, and that 

 would destroy the dark field, as the only light 

 entering the objective should be sent to it by the 

 reflecting, refracting, or dispersing action of the 

 object. For all dark-ground work the light should 

 be brilliant, and for high-power work it must be 

 very brilliant. If sunlight is available, that is 

 good. Of the artificial lights the small arc lamp 

 (fig. 49) is most satisfactory. A concentrated 

 filament nitrogen-filled mazda lamp of 100 or at 

 most 250 watts is also good. 



From the great obliquity of light required for 

 high powers it is necessary to apply the immer- 

 sion principle to the condenser, for if the light in 

 the condenser is above 41 (the critical angle), it 



will not emerge from the terminal face of the P . Funnel-shaped 



reducing diaphragm 



condenser, but be totally reflected (Ch. IX). By screwed into the lower 



adding the homogeneous immersion fluid between .° VjS " boot " °P- 



the condenser and the microscopic slide the light 



passes on directly without change to the object, and some of it is 



directed by the object to the objective. The slides should be 1 mm. 



or less in thickness so that the light can focus on the object. High 



powers with dark-ground illumination are much used at the present 



time in the study of living microbes; and in zoology, embryology, and 



histology it opens up a promising method of demonstrating minute 



granules in living things, the movement of cilia, amoeboid movements 



of amceba, of leucocytes, and the Brownian movement of the granule, 



in leucocytes, salivary corpuscles, etc. 



Fig. 4S. High- 

 power Objective 

 with Aperture Re- 

 ducing Diaphragm 

 for Dark-ground 

 Illumination. 



(From Chamot). 



