CRITICAL LIGHT AND IMAGE 



103 



Before concluding what we have to say concerning the substage condenser, we 

 must explain how we can place its optical axis coincident with that of* the microscope. 

 In other words how to place it in such dj position that a line drawn through the axis 

 of the eye-piece and objective shall be coincident with the axis of the microscope. The 

 importance of so doing is very great, and when it is shown how to take a photograph 

 it will we seen that this point must be very carefully looked to or the best of defini- 

 tion and illumination will be lost. This is quite an easy matter with low-power 

 objectives, but requires care with the high-power ones. 



Having focussed the object (so as to get the objective in its usual position) the 

 iris diaphragm is closed to its smallest extent, and lowered with the condenser until 

 its magnified image appears in the eye-piece. With the adjusting screws it is then 

 centred. When using high powers such as an ^th and xVth, it is safer to centre Jirst 

 with a half-inch, because then the error in centreing for the high objective will not 

 be so great as to cause the minute aperture of the iris diaphragm to lie out of the 

 field, which may easily be the case if the condenser happened to be much " out of 

 centrality." The same performance is gone through afterwards with the high-power, 

 but it should be remembered to ahvays use a low eye-piece at first, because with a 

 very high one the image becomes too magnified. A point to recollect on centreing 

 is never to bring the iris and the condenser up, it should, we believe, without exception 

 be ahvays down. If this mistake be made, the top glass of the condenser strikes 

 against the specimen slip, and that against the front lens of the objective, perhaps 

 cracking all three, besides breaking the cover-glass and ruining the specimen. 



(ii) Critical Light and the Critical Image. — "Critical light" is a term 

 used to imply that the image of the illuminant is focussed sharply on the specimen. 

 We have referred to it before, but it remains yet to be spoken of in its entirety. For 

 many years microscopical observers did not perceive the importance of the remark 

 made by Brewster so long ago, " that the best performance of the objective cannot 

 be obtained unless the condenser is focussed on the object." We now proceed to 

 explain how this is to be done. 



Having focussed the object with tlie objective in the ordinary manner the 

 substage condenser is racked up or down until the image of the edge of the lamp- 

 flame or the lime is seen lying across the field ; as now the condenser and the 

 objective are both focussed on the object, critical light is said to be obtained. There 

 are those who strongly object to this critical " flame image," as it is sometimes called, 

 and will lower their condenser just enough to avoid it, but if this be done with high 

 powers the resulting gain in even illumination must only be obtained at the expense 



