492 DUNCAN J. REID ON CRITICAL ILLUMINATION IN 



with the concave mirror, without a condenser the light is very 

 feeble, and only a small part of the back of the objective is 

 illuminated. 



Moreover, an aplanatic achromatic condenser is advisable. 



The potveQ" and ]\'^. A. of the condenser should be selected to suit 

 the objective in use, i.e. it should be of about the same power, and 

 of at least the same aperture. If of much too short a focus 

 relative to the power of the objective, it may be difficult, if not 

 impossible, to fill the field with light. If its N.A. is not at 

 least equal to that of the objective, the full aperture of the latter 

 cannot be filled with light. It may be necessary, unless a com- 

 plete battery of condensers is available, to reduce the power of the 

 high-power condenser by removing the front combination. This 

 does not always give us a perfect condenser, unless it has been 

 constructed to be employed in that way ; but with those that I 

 have so used I have obtained very good results. It is, besides, 

 only with low powers that this has to be done. 



It may here be observed that condensers of the continental type, 

 with large lenses, are a little more troublesome to manage than 

 the small English forms, as with the former the lamp and 

 collecting lens have to be removed to a greater distance from the 

 microscope in order to fill their full apertures with light. 



5. Distance of Lamp from Suhstage Mirror. 



It will be found that this has to be varied, according to the 

 power and N.A. of the objective and substage condenser, and 

 also according to the focal length of the collecting lens. 



In photomicrography, where it is convenient to keep the lamp 

 always at the same distance, the difficulty is got over by the use 

 of supplementary lenses. This, however, would be inconvenient for 

 visual work. 



Keeping in mind that the nearer the lamp and collecting lens 

 are to the microscope, the larger is the illuminated area of the field, 

 and that the farther they are away, the larger is the image of 

 the lamp flame projected on the mirror, and so available for filling 

 the condenser, it is evident that we have to find a middle distance 

 at w4iich both field and back combination of objective are filled 

 with light. It is quite common to see a man using an immersion 

 objective with a large aperture, with his lamp and collecting lens 

 only a few inches away. The field is fully illuminated, but, if tbe 



