A. A. MERLIN ON CRITICAL EMPLOYMENT OF THE MICROSCOPE. 203 



are opposed to critical large-cone illumination, and advocate in 

 its place the employment of narrow incident pencils. To such 

 as wish to examine the general question of its soundness, I 

 would earnestly recommend the perusal of Mr. J. W. Gordon's 

 recent brilliant paper on the whole subject.* 



1 am aware that many supporters of the diffraction theory, 

 being not only theoretical but experienced and skilled practical 

 microscopists, have now tacitly admitted the value of large-cone 

 illumination, without apparently realising the wide gulf that such 

 an admission has opened between their views and those of the 

 author of the theory. In this connection it is instructive to 

 notice that the deductions contained in the clear summary of 

 the diffraction theory in the seventh edition of " Carpenter," 

 published in 1691, which is peculiarly authoritative and weighty 

 as having been submitted to Professor Abbe himself, and as 

 having obtained his cordial endorsement, together with the 

 expression of his greatest satisfaction at seeing his " vdews 

 represented in the book so extensively and intensively," f have 

 not been in any way modified in the eighth edition, published ten 

 years later. This, taken in conjunction with the significant fact 

 that the best substage condenser constructed by the firm of Zeiss 

 for use with the highest powers still only affords a maximum 

 aplanatic cone of N.A. "65, clearly indicates that, whatever 

 admissions or changes of front his self-constituted supporters and 

 defenders may have thought fit to make. Professor Abbe himself 

 continues to unwaveringly maintain that strictly similar images 

 are alone to be expected with the central narrow incident pencil, 

 this being the necessary condition for the admission of the whole 

 of the diffracted light. What more logical conclusion could be 

 possibly reached on the hypothesis that microscopical vision is sui 

 generis, and that " the images of minute objects are not delineated 

 microscopically by means of the ordinary, laws of refraction ; they 

 are not dioptrical results, but depend entirely on the laws of 

 diffraction " ?5: Undoubtedly, on any such assumptions as these, it 

 is reasonable to maintain that the nearest possible approach to 

 truth in the rendering of minute structure will be only attained 



* " An Examination of the Abbe Diffraction Theory of the Microscope." 

 Journal R.M.S., 1901, page 353. 



t Preface to Carpenter's " The Microscope," 7th edition, p. viii. 

 X Carpenter's " The Microscope," 8th edition, p. 62, 



