78 BACTERIOLOGY. 



be concentrated on that part of the microscopical preparation which 

 requires most careful study (Fig. 23). 



To obtain the best definition considerable time must be spent in 

 the arrangement of the illumination. The lamp and microscope 

 having been placed in position, a low power is first used and the 

 smallest diaphragm. On looking through the microscope it will 

 probably be observed that the image of the diaphragm is not in 

 the centre of the field. By moving the centring screw of the con- 

 denser this may be adjusted. The image of the edge of the flame 

 may not be central, and this must be adjusted by moving the lamp 

 into position. The low power is then replaced by a high power, 

 the largest diaphragm used, and the bacteria brought into focus. 

 The diaphragm must now be replaced by one of medium size, and 

 by racking the condenser up and down, a point will be arrived at 

 when the image of the edge of the flame appears as an intensely 

 bright band of light. If this is not exactly in the centre of the 

 field the centring screws of the condenser must again be adjusted. 

 Lastly, by trying different sizes of diaphragms, and focussing with 

 the fine adjustment, and using the correction collar, we arrive at the 

 sharpest possible image of the bacteria. 



When the condenser has been accurately centred, it will still be 

 necessary to focus it for each individual specimen, so as to correct 

 for difference in the thickness of slides and the layers of mounting 

 medium. Correction for different thickness of cover-glasses must 

 in each case be made by means of the collar adjustment in the follow- 

 ing way. A high-power eye-piece is substituted for the ordinary 

 eye-piece, and the fault in the image will thereby be intensified. By 

 moving the collar completely round, first in one direction and then 

 the other, while carefully observing the effect on the image, it will 

 be seen to become obviously worse whichever way the collar is turned. 

 The collar must then be turned through gradually diminishing dis- 

 tances until an intermediate point is reached at which the best image 

 results with the high-power eye-piece, and on replacing this by the 

 low-power eye-piece the sharpest possible image will be obtained. 



Effect of the sub-stage condenser. The sub-stage condenser 

 gives the most powerful illumination when it has been racked up 

 until it almost touches the specimen. It produces a cone of rays of 

 very short focus, and the apex of the cone should correspond with the 

 particular bacterium or group of bacteria under observation. The 

 effect of the condenser without a diaphragm is to obliterate what 

 Koch has termed the structure picture. If the component parts of a 

 tissue section were colourless and of the same refractive power as 



