834 SUMMA.RY OF CURRENT RESEARCHES RELATING TO 



lished in August 1878, support, the author contends, the position 

 taken by himself, Mr. Dallinger's description of the penetrating 

 power and fine definition of the lens, is evidence that a superior 

 objective for delicate original work, requires penetration as well as 

 definition. 



The practical conclusion is, that neither penetrating objectives nor 

 defining objectives are alone sufficient for all classes of microscopical 

 investigation ; but both kinds are needed, of all the powers, and if 

 the microscopist is limited in the number of his lenses, he will find 

 the widest capabilities in the low-power defining, and high-power 

 penetrating, objectives. The paper closes with the recommendation 

 that opticians should endeavour to secure the best possible combina- 

 tion of defining power, with penetration, in the same objective. 



Mr. Vorce further writes on the same subject :* — 



" I partly agree with Dr. Blackham and his followers, but my 

 views may be stated thus : Penetration is antagonistic to perfect 

 definition, and, therefore, penetration is an objectionable quality in 

 objectives whose purpose is definition solely. But mere definition is 

 not the sole purpose of objectives, and in those objectives whose pur- 

 pose is comprehensive view, penetration is a virtue as well as is 

 flatness of field. Fortunately penetration cannot be got rid of by 

 the opticians, except at the expense of the field of view ; and 

 Dr. Blackham's 1-inch objective still retains some of it, while it has 

 gained much over the narrow angles in definition, and in this respect 

 his objective approaches somewhat to the capabilities of that wonder- 

 ful optical instrument, the human eye, which has hoth penetration and 

 definition in a very great degree, in consequence of its inimitable 

 ' compensating adjustment,' so well described by the Doctor. In 

 some objectives this faculty of penetration is very striking. I 

 remember distinctly with what surprise I saw, long ago, under a 

 1-inch of first-class English make, a stained specimen of Utricularia. 

 At one view were seen the top layer of cells, the spiral vessel occupy- 

 ing the middle of the stem, and the bottom layer of cells ; and all 

 these with so much distinctness that, while the threads of the spiral 

 vessel were in actual focus, the cells above and below were so nearly 

 in focus that there was no dimness to their outline, and without 

 changing the focus the size, shape, depth, and arrangement of the 

 cells of the stem could be clearly seen, and the structure and central 

 position of the spiral vessel perfectly seen. I am not ashamed to say 

 that to this day I long to possess that objective, which, according to 

 Dr. Blackham, is so faulty." 



Mr. Vorce also takes exception to the figures of wire netting which 

 we referred to at p. 887, contending that they are fallacious as shown 

 on paper. As given, the figures " show exactly what a non-penetrat- 

 ing objective would show, viz. : all the lines equally defined ; but 

 with a penetrating objective one set of lines would be a little clearer 

 than the other, and although there would be a slightly poorer defini- 

 tion of the lines in focus, there would be sufficient definition of both 



* Amer. Joum. Micr., v. (1880) pp. 183-4. 



