204 A. A. MEKLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 



With a narrow central illuminating pencil, for we can all easily 

 satisfy ourselves by experiment that the strongest and most 

 marked diffraction images and effects are thus obtainable. As 

 we increase the diameter of the illuminating pencil we can experi- 

 monUilly observe that the pxture rapidly becomes worse and 

 worse from a diffractive point of view until, when we reach a |^ths 

 or liths cone, no certain diffi-action effects are discernible. In 

 the case of a delicate object possessing fine structure we have an 

 ima*j:e «rreatlv lackinij: contrast, and exhibitini:: no broad diffrac- 

 tion edges, in which we may glimpse points just within the 

 theoretical grasp of the objectiv^e, the ditliculty of holding these 

 for any length of time strongly reminding a telescopist of his 

 fleeting glimpses of planetary detail just visible under the most 

 favourable atmospheric conditions. This is precisely the contrary 

 of what we should expect to see according to the deductions from 

 the diff'raction theory summarised in '* Carpenter," for these 

 would make us feel confident that in the instance of the 

 ^■1. pelhicida the central narrow incident pencil should produce 

 the most truthful picture, and one more nearly approxima- 

 ting to the ultimate structure in exactness, than would be the 

 case with a large pencil. But we have observationally found 

 that with N.A. '985 and a large ^ths or yths cone, with light 

 passed through a copper acetate screen, the striae, running at 

 abaut 9-JI:,000 to the inch, are discernible and clearly separated, 

 while they are absolutely invisible with a much narrower pencil. 

 Xow the Abbe limit of resolution for N.A. "99 being 95,4-40 lines 

 to the inch with oblique white light (Line E), and 103,458 with 

 blue light (Line F), therefore, if this observation is correct, we 

 may feel confident that it is not the narrow, but the very large 

 axial cone, which affords the maximum resolution, and the nearest 

 possible approach to the complete rendering of the object that the 

 aperture of the objective will permit. 



This conclusion is of paramount importance to the non- 

 theoretical practical worker, for it is only on the supposition that 

 the large cone affords a critical or true image, that he can place 

 reliance on the objective reality of any minute features that 

 ha}>pen to be visible in his instrument. He knows, of course, 

 that the ultimate structure is probably so minute as to utterly 

 elude the grasp of his lenses ; but the supremely vital point in 

 practical work is to feel assured that what is visible should, so 



