ZOOLOGY AND BOTANY, MICROSCOPY^ ETC. 319 



82°. In the one case the light comes with an obliquity of 90^, whilst 

 in the other case it has only an obliquity of 41°, and the object can 

 of course be seen more completely and distinctly by reason of the 

 greater obliquity.* It cannot matter whether the medium is air or 

 balsam; the obliquity per se is obviously not altered in the least 

 degree by the change of medium," 



For years the ablest and most experienced microscopists in 

 England, and indeed everywhere, accepted the doctrine without 

 question that there teas a special virtue in the increased obliquity of 

 the light incident on and emanating from the object, and not only so, 

 but pointed out the reason for the (supposed) fact, explaining it to 

 depend upon what were termed " shadow effects," i. e. in the same 

 way as the inequalities on the face are better brought out by oblique 

 than with direct light. This process of discovering a reason for a 

 supposed fact prior to any verification of the fact itself, is only 

 paralleled by the famous problem said to have been propounded by 

 Charles II. to a learned Society — " Why does a vessel of water with 

 a fish in it weigh uo more than it did when there was no fish ? " 



About thirteen years ago it occurred to Professor Abbe to in- 

 vestigate the reason for the supposed value of obliquity qua obliquity, 

 and he naturally proposed in the first place to consider anew the 

 grounds on which the view had been based when originated. To his 

 surprise he found that no attempt had really been made to investigate 

 the matter ; that there was no theory and no experiment to support the 

 alleged fact, which had been quietly assumed by every one to be a 

 fact, no one knew how, except from some fancied analogy to ordinary 

 vision, regardless of the different conditions of microscopical vision, 

 or probably from incomplete generalization from the fact that a 

 pencil of 170° does show minuter structure than a pencil of 80° in 

 the same medium ; but that, like the fish problem, it had not occurred 

 to any one that the task of verifying the existence of the assump- 

 tion should have preceded any reasoning upon it or attempts to 

 explain it. 



A long course of experiments extending over several years was 

 undertaken by Prof. Abbe, which established the fallacy of the old 

 view, and by force of the necessity for explaining intelligibly the 

 real specific function of increased aperture, led to the enunciation 

 of the most important theory that has ever been propounded in regard 

 to the Microscope itself, viz. the Abbe theory of microscopical vision 

 to which we refer hereafter.f 



The cardinal point in Prof. Abbe's experiments was the discovery 

 that the utilization of increased aperture depends not on the obliquity 

 of the rays to the object (as had been assumed), but on their obliquity to 

 the axis of the Microscope. 



* Sometimes the opposite view is put forward, viz. that by the increased ansle 

 of aperture a less perfect image of the object is obtained in consequence of the 

 unnatural character of vision with large angles, which is supposed to produce 

 distortion and indistinctness not found in ordinary vision with the naked eye 

 where small-angled pencils are in question. 



t See IV., infra, p. 347. 



z 2 



