USE AND MANIPULATION OF MICROSCOPE 149 



getting an evenly illuminated picture as already described with 

 a low-power objective on the microscope. 



This experimental method of observation of the projected 

 microscopic image may then be extended farther, and a rather 

 higher power objective put on to the microscope say, a half- 

 inch the sub-stage condenser replaced, and the illumination 

 watched as before. The upper lens of the sub-stage condenser 

 should be removed, if its construction will admit of that, so that 

 the image of the illuminant is larger and covers a larger field. 



The illumination in this case may be best effected by 

 removing the bull's-eye for the moment, then racking up the 

 sub-stage condenser until an image of the illuminant is seen 

 projected over the image of the object. The illuminant should 

 in this case be about ten inches from the microscope sub-stage. 

 This image of the illuminant should be seen quite in the centre 

 of the field. Then again interpose the bull's-eye condenser, 

 and arrange it as before, and in such a position that the field 

 again becomes evenly illuminated. The effect of altering the 

 position of the bull's-eye between the light and the microscope 

 should be noted ; but the change should be effected by 

 moving the bull's-eye from the position in which it gives 

 a parallel beam towards the microscope, so that it giveg a 

 converging one. To obtain an evenly illuminated field it is 

 necessary in fact for an image of the aperture of the bull's-eye itself 

 to be projected into the object-plane. In cases where the bull's- 

 eye has an iris-diaphragm or a single metal plate with a small 

 hole in it an accessory often supplied with these appliances- 

 it is easy to focus the image of this small aperture in the same 

 plane as the image of the object by racking up or down the 

 sub-stage condenser. On removing the stop the object will 

 then appear evenly illuminated. If a parallel beam from the 

 bull's-eye does not give enough light, a convergent beam may 

 be used ; but it should be so arranged that a roughly 

 focussed image of the radiant is seen on the iris-diaphragm at 

 the back of the sub-stage condenser. The illumination will 

 then be sufficiently critical for use with an objective not higher 

 than half an inch. 



Further reference to this and instructions in detail will be 

 given later when referring to the photography of typical objects ; 

 but this preliminary observational work will be found most 



