360 SUMMARY OF CURRENT RESEARCHES RELATING TO 



of 180"^ angular in air, that is in excess of the maximum to which 

 a dry objective can attain, so that the contrary view must now be 

 ranked with the belief in the flatness of the earth which still lingers 

 in places. 



Second. Whilst it is conceded by all parties that increase iu 

 aperture is accompanied by an increased efiect in microscopical vision, 

 the reason cannot be explained on the " angular " view. When it is 

 seen, however, that microscopical vision is not, as erroneously sup- 

 posed, subject only to the same conditions as ordinary vision, the 

 explanation of its true character is shown to lead to the establishruent 

 of the diffraction, in lieu of the dioptrical, theory. 



Thirdly. Dry objectives must be considered to represent an im- 

 perfect phase of construction so far as regards the delineation (not 

 " resolution " merely) of very minute objects. Dry objectives are and 

 will always be of the highest practical value in those lines of research 

 in which low and moderate apertures are sufficient for the perfect 

 delineation of the objects under investigation. As soon, however, 

 as minute objects are observed which require high amplifications in 

 order to be seen, dry objectives cannot compete with immersion inas- 

 much as high apertures are necessary in order to obtain perfect 

 delineation. No improvements in construction, therefore, can render 

 a dry objective, whose aperture is limited to 1*0, equal for the 

 delineation of minute objects to a wide-angled immersion objective 

 whose aperture is limited only by the higher figure which represents 

 the refractive index of the denser immersion medium. 



For any of the purposes, therefore, for which the delineation of 

 very minute objects is required in the Microscope — and it need hardly 

 be said that this is by no means all purposes — a perfectly constructed 

 wide-angled homogeneous-immersion objective is necessarily to be 

 preferred to all others, for the simple reason that it enables us to see 

 more, and more perfectly. 



It will be recognized that this is not a merely abstract result, of 

 interest to no one but the optical mathematician, but that it is a 

 practical result in the fullest sense of that term. 



