USE OF AUXILIARY CONDENSER 147 



to the plate to be used. Carefully examine the image 

 for specks of dust on lenses or slide, and to see that the 

 condenser aperture is correct. Place the colour screen in 

 its position and carefully focus ; if apochromatic lenses 

 are used focus can be found in white light and then the 

 screen adjusted, but this must not be done with achro- 

 matic objectives. It is as well to focus finally on the 

 clear glass, for which purpose a focussing lens must be 

 used, and held in such a position that the fine cross marked 

 with a diamond on the inside of the screen is in focus. 

 As the image is liable to go out of focus on account of 

 vibrations or the heat of the lamp, it should be examined 

 again before inserting the dark slide if there is any delay 

 before exposure. 



9. Exposure. Place the dark slide in the camera 

 without shaking the latter, place a piece of card between 

 the microscope and lamp, draw the slide and move 

 the card aside for the time of exposure. Replace the 

 card and slide. During the exposure allow as little 

 extraneous light as possible to fall on the lens or object. 



ILLUMINATION WITH AUXILIARY CONDENSERS 



Parallel Light. Operations 1 to 5 are carried out as 

 already described. 



6a. Adjustment of Paralleliser. The auxiliary conden- 

 ser or paralleliser is placed with its plane or concave side 

 towards the lamp, and between it and the microscope, 

 with the radiant at its principal focus. It is correctly 

 placed when it appears quite evenly full of light, and 

 projects a perfectly even disc on to the substage condenser. 

 To determine whether the beam is parallel blow tobacco 

 smoke across it, or move a white card backwards and 

 forwards between the paralleliser and microscope, also 

 examine the disc projected on the card for any uneven- 

 ness. 



