

piOLOGY 

 UBRARY 



tion of the bacteria. Also, the apertures of the extreme rays a 1 

 and /j'PYie within the limits 1.1 and 1.45, from which it will be 

 seen that a considerable amount of light is collected at 'P. When 

 dry lenses are used all the rays which enter from below and 

 converge towards P go to illuminate the bacteria, as shown by lines 

 and dots, and are totally reflected at the surface of the cover-glass. 

 The light diffused by the bacteria, as represented by dotted lines, 

 enters the objective and thus produces an image of the bacteria, 

 which under these circumstances behave as self-luminous bodies. 

 Since the rays are united at P by reflection instead of by refraction, 

 there is no chromatic dispersion, and the annular illumination of 

 the bacteria obviates diffraction. 



The light-gathering power of this reflecting condenser is such 

 that with an arc lamp fed by a continuous current of 4 amp. the 

 intensity of the illumination suffices for the photography of living 

 bacteria. 



The optical portion of the dark -ground illuminator is contained 

 in a mount provided with a centreing arrangement and slips from 

 below into the sleeve which usually carries the Abbe condenser. 



Since the point P should lie within the preparation it is neces- 

 sary to use slides of uniform thickness, not exceeding 1.0 mm. 

 The requisite correction is effected by raising or lowering the dark- 

 ground illuminator by means of the movement forming part of the 

 illuminating apparatus. It should however be noted that the space 

 below the object slide should always be filled with oil. 



The best source of light for the dark-ground illumination is 

 furnished by a small arc-lamp, but where this is not available a Nernst 

 lamp or incandescent gas lamp may be used, although, naturally, 

 the illumination will not be so bright. 



The object may, as stated above, be viewed with dry lenses, 

 the illuminating rays being totally reflected from the upper surface 

 of the cover-glass, whereas only those rays which are dispersed by 

 the bacteria are allowed to enter the objective. High powers of the 

 dry series are better adapted for this purpose than lower powers, 

 partly owing to their higher apertures and their consequent greater 

 light gathering power, and partly owing to their greater magnifying 

 power. Owing to the influence which the thickness of the cover- 

 glass exercises upon the performance of high power dry lenses, it 

 is essential to employ cover-glasses of the proper thickness, viz. 



