196 A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 



rudimentary elements of critical microscopy, notwithstanding 

 the fact that the instruments and accessories actually employed 

 could be easily utilised so as to afford far more satisfactory 

 results. The following causes should perhaps be reckoned 

 amongst those principally responsible for the existence of this 

 state of things. 



(1) The considerable training and practice required to enable 

 the eye to fully grasp the points of difference between a critical 

 and non-critical diffraction image, or to appreciate the delicately 

 faint, but true, rendering of all the visible features in the former 

 case, as compared with the misleading and unreliable, although 

 well-marked and obtrusive, diffraction outlines and effects 

 invariably associated with the latter. 



(2) The strong prejudice evinced by many workers in favour 

 of an evenly-lighted "full moon" field in place of the sharply- 

 focussed image of the light source. It is true that with a low- 

 power substage condenser the focussed image of the lamp flame 

 can be made to fill the entire field, but the rough-and-ready 

 worker finds it much simpler to secure this result by lowering 

 or raising his condenser. 



(3) Carelessness and apathy regarding microscopical manipu- 

 lation ; it being considered that the labour requisite for the 

 acquirement of even an elementary knowledge of the subject 

 would be a mere waste of time on the part of a naturalist or 

 kindred worker. 



(4) A conscientious objection to the critical large cone image 

 on theoretical grounds ; for, according to the Abbe dictum, 

 " Strictly similar images cannot be expected, except Avith a 

 central illumination with a narrow incident pencil, because this 

 is the necessary condition for the possible admission of the whole 

 of the diffracted light." * 



I will ask your indulgence to here examine each of these 

 points in detail. 



With regard to the first, nothing but practice and patient 

 work will enable the microscopist to perceive in a delicate object 

 all the minute faintly-outlined features just within the resolving 

 power of the objective employed, and revealed by it under the 

 action of a solid |-ths or even iths axial cone from a well-corrected 



* Carpenters " The Microscope,' 8th edition, page 75. 



