338 SUMMAKY OF CURKENT RESEARCHES RELATING TO 



in performance the best immersion glasses, which would undoubtedly 

 be a great triumph, inasmuch as dry objectives are so much more 

 convenient to use than immersion. Why should not this be possible, 

 he thinks, with skill and patience, if the question depends simply on 

 the angle of the admitted pencil ! 



(3) If, again, he should hold one of the views of the effect of 

 increased angle which we have above refen-ed to (p. 319), viz. that 

 it is a defect, he will confine himself to the construction of objectives 

 of low angle, which he contends must in all cases exhibit objects better 

 than those of large angle. With the large angle there is greater all- 

 round vision, as if many eyes were looking at the object at the same 

 time, and hence the resultant effect must, he thinks, be general 

 indistinctness and imperfection in the image. Yet objectives of the 

 highest angles hitherto constructed are easily proved to give images 

 of sufficiently thin preparations without confusion. 



(4) Several other practical mistakes flow from the same erroneous 

 theoretical assumptions, one of the most important of which we deal 

 with under a separate head, " Not Image-forming Hays." 



(6) " Not Image-forming Rays." — This represents a very curious 

 form of the angular fallacy. 



At the commencement of the old aperture controversy, it was 

 asserted that no objective could be constructed, whether dry or im- 

 mersion, which would allow of an angle in excess of 82° in the body 

 of the front lens. In the same way as Nature abhorred a vacuum, she 

 was supposed to abhor an objective which could act in such defiance 

 of established laws of critical angles and otherwise ! 



At the time that this view was first enunciated, no such objectives 

 had in fact been constructed, so that the statement was then partially 

 excusable. The first claim to have constructed such an objective 

 was received with great derision, and it required some years to finally 

 establish what is, however, now universally accepted as a fact, that 

 immersion objectives do allow of an angle in excess of 82° in the 

 front lens. 



The germ of the original fallacy still persists, however, in the 

 contention that although the surplus rays in excess of the 82° do 

 really exist, yet that they are not image-forming rays. 



Now as the limit of 82° is simply double the critical angle between 

 air and glass, it can have no possible application whatever to the case 

 of rays passing, not from air, but from water or oil to glass, and it 

 must be clear therefore on theory that this notion is a fallacy. 



It is obviously for those who contend that the rays (even when 

 passing from oil to glass, and without any air-film!) cease to be 

 image-forming rays at 82°, to show on what grounds they base their 

 contention ; but we will nevertheless point out one or two experiments 

 which — apart from any reference to the obvious theoretical conside- 

 rations — show the effective action of the surplus marginal rays with 

 wide-angled immersion objectives. It is hardly necessary to point 

 out that for these experiments it is essential to use objectives which 

 are properly corrected for all zones, marginal as well as central. There 



