USE AND MANIPULATION OF MICKOSCOPE 173 



that is projected into the field of view. Variation within some 

 limits is therefore possible, depending on the particular sub- 

 stage condenser. 



Whatever auxiliary condensers are adopted, it is most im- 

 portant to see that they are so adjusted that no reduction 

 in the available working aperture of the sub-stage condenser 

 results from their use. This may be determined by first obtain- 

 ing a critical image, in the manner already described, without 

 any optical system intervening between the source of light and 

 the microscope. Then remove the ocular and see, by looking 

 at the back lens of the objective in the manner also previously 

 described, what proportion of the objective- aperture is actually 

 being used. Now arrange the auxiliary condensers in the 

 manner required ; there should then be no decrease in the 

 working aperture as seen by looking at the back lens of 

 the objective. Whatever auxiliary lens is used, it must also 

 be seen that the back lens of the sub-stage condenser is, at 

 least for high powers, perfectly filled with light. 



The proper use of these auxiliary condensers is a matter of 

 considerable moment, and those who are not familiar with them 

 are strongly advised to study, by the method already set out, 

 the effect of altering their position, i.e. by observing a projected 

 image on a white reflecting-surface. More information will be 

 gained by some preliminary experimental work of this character 

 than by reading pages of description. The worker should 

 set up his various parts in the manner described and note 

 the results, and should then make some variations in the 

 relative positions of these parts and see exactly what happens. 

 It is as important to be able to recognise any error that may 

 creep in, and to be able to determine its cause, as it is to set up 

 the apparatus with precision and in proper working adjustment. 

 It is necessary to see that the iris- diaphragm, or any stop used 

 as its substitute forming a part of the auxiliary condensing 

 system, does not affect the working aperture of the sub-stage 

 condenser or the objective, assuming, that is, that the beam 

 is sufficiently large to fill the sub-stage condenser with light. 

 The iris-diaphragm should only control the effective size of the 

 image of the illuminant, and of the field of view. In all cases 

 where this image of the diaphragm is the critical image f ocussed 

 in the field of view, it is advisable to close it to such an extent 



