290 SUMMAEY OF CUKEENT EESEARCHES RELATING TO 



and these dimensions really constitute a still more minute set of lines, 

 whose spectra are far beyond the collecting power of our lenses. 

 Hence the reason why real lines, like Nobert's test-bands, can be truly 

 " resolved" or photographed, though microscopic objects cannot. 

 Supposing the objective can resolve lines of 100,000 to the inch, all it 

 is capable of showing in an object is, that something — some variation 

 of structure — occurs at regular intervals of 100,000 to the inch. If 

 this is so, it will be shown as lines ; but it is perfectly obvious that 

 very rarely can it really he mere lines. It will have form of some 

 kind ; and every minute variation in light and shade due to that form, 

 constitutes an infinitely minute set of distances, which will all cause 

 their own spectra, too distant, if not too faint, to be gathered in. 



Error may arise in yet another way. Lord Rayleigh has shown 

 mathematically, and partially proved experimentally (by gratings eaten 

 out in gelatine) that if an optical grating be composed (instead of white 

 and black lines) of equally transparent narrow stripes, whereof each 

 alternate one retards light half a wave-length more than its neighbours, 

 the spectra are fourfold in brightness. Now these also would image lines, 

 though falsely. The application to transparent microscopic objects 

 need not be pointed out. 



Yet the more we understand the true relations of these spectra, 

 and their optical effects, the truer will our interpretations become, 

 within, at least, the limits of microscopic vision, and the more certainly 

 shall we be directed to methods of manipulation which may truly 

 interpret the phenomena. Taking merely the theory of the matter, 

 let us consider the case of fig. 41 where only the lines A A and B B 

 really exist, but C C are apparent by the illusory spectra of the 

 narrow spaces between A B, A B. It is plain we have means, if our 

 suspicion as to the existence of C C C is awakened, of clearing it up. 

 For we can stop off not only the central pencil, but those inner spectra 

 which give us the coarsest intervals AAA. This will bring into 

 stronger relief the spectra representing the next widest spaces, B A, 

 B A, and thus we may correct the former result. Also it is obvious 

 that any skilful manipulator with a knowledge of physical optics, by 

 stopping off central pencils, and, when necessary, inner spectra, 

 might bring into stronger relief the much fainter spectra caused by 

 fainter and finer striations, which were before " drowned," as it were, 

 in the coarser phenomena. Mr. Stephenson proved this in the case 

 of P. angulatum, bringing out minute patterns, when the central 

 pencil was stopped off, which had never before been seen. 



It is also evident why so much is learnt from various incidence 

 of the light ; but it will also appear that this should be studied more 

 gradually than most mirror arrangements permit. As a rule, micro- 

 scopists adjust for one obliquity, and make their observation ; then 

 try another. It would rather appear that continuous observation 

 under a steadily increased obliquity must be necessary to good inter- 

 pretation ; and that even then a competent knowledge of the physical 

 phenomena of optical gratings is necessary, as well as a careful 

 collation and comparison of the appearances with those presented 



