104 PHOTO-MICROGEAPHY 



of critical definition. When searching for objects with low powers, say, with an inch 

 or half-inch, it may be at times more convenient to sacrifice the ideal image to obtain 

 an evenly illuminated field ; for when the correct position of the object is found, it is 

 simple enough to re-obtain the critical image for purposes of study. If equal critical 

 illumination, however, is required absolutely over the whole field when visually 

 employing the microscope, it can be obtained, if the condenser is really aplanatic, by 

 turning the wick of the lamp broadways after it has been focussed on the object ; but 

 if the condenser be not so aplanatic as could be desired, or ivhen taking a photograph, 

 a second condenser (the best we know being called a Nelson's " Compound Bull's- 

 Eye ") must be interposed between the light and the mirror of the microscope, which, 

 when properly placed, will have the desired effect. 



As before stated, " critical light" should always be employed if the objective is to 

 perform at its best. There are optical reasons, too recondite to relate here, which 

 necessitate it absolutely and positively, and those who think differently must give 

 satisfactory evidence that they have carefully read the writings of Nelson and other 

 great authorities before any attention can be given to their theories. When " critical 

 light" is employed the resulting image is called " critical " also. 



(iii) The Auxiliary Condenser. — This usually consists of two lenses loosely 

 fitting into a ring of brass and supported by a stand which is supplied with a 

 centreing attachment contained in itself as shown in Fig. 56 by Watson. 



Personally we prefer having the lenses mounted like the limelight (Fig. 3, page 4) 

 used by us, which has already been fully described, page 5. The arrangement is shown 

 in Fig. 57, where the milled heads for raising and lowering are seen. An " iris " cap 

 is also displayed, which fits over the lenses and reduces their aperture to a point of 

 light when required. 



The best position for this auxiliary condenser is such that it will throw parallel 

 rays on the mirror, or if the mirror has been removed, on to the substage condenser. 

 This is obtained by pushing it quite near the illuminant — the round side towards the 

 light — so that the rays issuing from it are practically the same width at a distance of 

 a few feet. A puff of smoke from some brown paper " half alight " will make their 

 path very apparent and show them up admirably. If they widen, the condenser is too 

 near the illuminant ; if they come to a focus, too far away. 



In some cases of high magnification we have found definition sufi'er decidedly from 

 the use of this second condenser, whilst with other objects it was exactly the reverse! 

 There are those better versed in mathematical optics than ourselves who declare that 

 the final definition 7nust always suffer unless this condenser be corrected just as well as 



