166 A MANUAL OF BACTERIOLOGY 



the centre of the microscope mirror. The light, bull's-eye 

 and centre of the mirror should be in strict alignment, 

 the plane of the bull's-eye cutting the line joining the 

 centres of the radiant and mirror at right angles. Having 

 so set up the illuminant and microscope, remove any 

 condenser and objective and introduce a low-power 

 ocular. The light is then adjusted with the mirror, so 

 that the eye-piece is evenly and completely filled with 

 light. This may be ascertained by looking down on the 

 ocular with the eye 8 or 9 in. above it ; when the adjust- 

 ment is correct, a perfectly even disc of light is seen. 

 Another method is to hold a piece of paper 2 or 3 ft. 

 above the ocular and observe the disc of light upon it. 

 If the disc of light in either case is not perfectly even, 

 there is some lack of centra tion, and this may then be 

 corrected by adjustment of the mirror or alteration of the 

 position of the light, for the lack of centration may be 

 due either to the beam not being projected along the 

 optical axis of the microscope (adjustment by the mirror) 

 or to the image of the source of light not falling on the 

 centre of the mirror (adjustment of light and bull's-eye). 

 This adjustment having been effected, the dark-ground 

 condenser is slipped into position in the sub-stage fitting 

 and a large drop of cedar-wood oil (or of glycerin and 

 water) is placed on the top lens. A low-power objective 

 is then attached to the microscope and the slide, with the 

 object, placed on the stage. To avoid air bubbles, which 

 is essential, a small drop of oil may be placed on the 

 under surface of the slide as well. Next, rack up the 

 condenser so that the under side of the slide is immersed 

 in the oil and the top of the condenser is nearly in contact 

 with it. The object is now brought into view by adjust- 

 ment of the objective and will be seen to be illuminated 

 with a disc of light. The dark-ground illuminator is then 

 focussed up or down until the smallest possible area of the 



