ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



289 



Aperture and Resolution.* — L. Wright, while agreeing in the 

 utter impossibility of ever knowing by absolute observation the " true 

 structure" of minute objects, yet thinks there is something in the 

 objections to overmuch dependence upon the results of very oblique 

 light. 



Let us suppose we have an object whose true structure is some- 

 thing like fig. 40, explained or described as follows: — 



A C B 



Fig. 40. 

 C A C 



C A 



Let the black lines AAA denote strong ribs, or striations, or ridges 

 20,000 to the inch ; B B lesser ridges midway between them, and 

 C C C either valleys or fainter markings midway between these. A 

 low-angled lens would show the black lines only ; a good glass would 

 bring out B B as well ; a good immersion the faint dotted lines C C C. 

 But even in this simple case these latter would certainly appear as 

 lines ; for the distances represented by the dots are far too minute 

 for their spectra (transverse to the others) being included in any lens 

 yet made. Let us, however, now suppose the real structure to be 

 modified as in fig. 41, the second strongest markings B B being next 



A B 



Fig. 41. 

 C G A B C 



G A 



AAA, but C C C altogether absent, and not as here shown. The 

 second lens would, in this case, show only a slight thickening or 

 blurring of the coarser striation AAA; but the immersion objective 

 might show nearly the same image as that given by the previous 

 structure, since the narrow intervals A B, A B, A B, would give the 

 same spectra as if the gaps C C C were filled up. It is true the wider 

 distances B A, B A, B A would, by their own spectra, strengthen the 

 B B lines ; but the dotted lines C C would be created, and appear 

 to show structure which did not exist. 



But no microscopic structure really consists of absolute lines, and 

 hence this is a very small part of the complicated problem. If, as we 

 have supposed, AAA are ridges, they will have some absolute breadth, 



* Engl. Mecb., xxxviii. (1884) pp. 470-1 (2 figs.). 



