76 JOURNAL OF THE [April, 



subject, Mr. Smith determined to try what could be learned by 

 the systematic and careful use of the Zeiss apochromatic lenses 

 under such conditions as to sub-stage illumination, by means of 

 a wide-angled condenser, as should give the new objective the 

 fullest scope for its power and quality. His conclusions from 

 this examination he has kindly laid before us, so fully illustrated 

 by beautiful lantern slides and prints that we can have little 

 ■doubt as to the appearances on which the conclusions are based, 

 except as to color. 



In such a case the real question is one of interpretation of 

 appearances seen under the microscope, and what 1 have to say 

 will bear chiefly on this point, with direct application to the study 

 of diatoms. 



All microscopists are acquainted with the position of Prof. 

 Abbe in regard to images formed by diffraction. As commonly 

 stated it amounts to a declaration that all microscopical images 

 •of structure with details smaller than. 0005 of an inch are diffrac- 

 tion images from which the true structure may be argued, but 

 which cannot be taken as in themselves true representations of 

 the structure. "The resulting image produced by means of a 

 broad illuminating beam," says Prof. Abbe {^R. M . S. Journal^ 

 December. 1889), "is always a mixture of a multitude of partial 

 images, which are more or less different (and dissimilar to the 

 object itself)." 



This theory has been very vigorously assailed by Mr. E. M. 

 Nelson, of London, from the practical and experimental side. 

 In a paper read before the Quekett Club in May last, entitled 

 " The Sub-stage Condenser: Its History, Construction, and Man- 

 agement; and its Effect Theoretically Considered," Mr. Nelson as- 

 serts that the cone of light from a sub stage condenser "should be 

 of such a size as to fill f of the back of the objective with light; 

 thus N. A. i.o is a suitable illuminating cone for an objective of 

 1.4 N. A." He says that "this opinion is in direct opposition to 

 that of Prof. Abbe," and to maintain it he denies the truth of the 

 diffraction theory as applied to microscopical images. He says 

 of it : "The diffraction theory rests on no mathematical proof — 

 in the main it accepts the physical law of diffraction; but on 

 experiment it utterly breaks down, all criticism is stopped, and 

 ■everything connected with it has to be treated in a diplomatic 



