ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 285 



least equals that aperture. If the Microscopes of the future should 

 utilize the refractive power of the diamond, all the objects would have 

 to be imbedded in diamond, without any intervening substance. The 

 result of this consideration is, therefore, that as long as aperture serves 

 that specific function which experiment and theory compel us to ascribe 

 to it at present, there is a limit to the further improvement of the 

 Microscope, which, according to the present condition of our knowledge, 

 must be considered as insurmountable."* 



It will be seen, therefore, that the diffraction theory, even before 

 the introduction of homogeneous-immersion objectives, took account of 

 apertures higher than 2, so that there is no foundation for the wildly 

 ridiculous suggestion that it is possible to " trace in all Dr. Abbe's 

 " subsequent papers the influence of two moods, and that at times he 

 " could not resist the evidence, as the aperture of the objectives became 

 " larger, that the image given by them was a truthful one." "f 



In addition to this it should be recalled that the first detailed 

 exposition of the diffraction theory on its final basis was published by 

 Professor Abbe in 1882. As homogeneous-immersion objectives were 

 made by Professor Abbe and Dr. Zeiss in 1878, it is quite a misapprehen- 

 sion to write that " since then has come the oil-immersion objective and 

 " the oil-immersion condenser, throwing a flood of light on the image 

 " not possible under the old methods, and what I cannot understand is that 

 " people should noiv revive the old doubts."X Whatever were the old doubts 

 they still remain in the same position — unchanged and unremoved by 

 anything that has happened since they were first shown to exist. 



(2) The second point with which we will deal is contained in a 

 statement the text of which is as follows : — 



"... Even Dr. Abbe seems to be frightened at the logical 

 outcome of his own theory, for further on he says, ' It is obvious 

 that a perfect fusion in every case of the same diffraction images, 

 and then an exact superposition of the resultant diffraction image 

 upon the absorption image, is only possible when the objective 

 is uniformly free from aberration over the whole area of its 

 aperture.' This clearly means that given perfect correction of 

 the objective there is perfect definition of the object, which to 

 me seems to contradict the former part of the paper." § 

 The misunderstanding here arises from not comprehending the 

 difference between the defining and the delineating power of an objec- 

 tive. 



Take, for example, the case of an objective which has an aperture 

 sufficient only to take in the first set of spectra of Pleurosigma angu- 

 latum. If the objective is perfectly corrected we shall have perfect 

 definition of the image to lohich those spectra give rise. But the objective 

 not having an aperture sufficiently large to take in the second set of 

 spectra will necessarily give a less perfect image than another 

 objective which takes in those spectra, and the first objective therefore, 

 though perfectly corrected and giving perfect definition of what it does 

 show, gives only an imperfect image. 



In the next number of the Journal we shall deal with further mis- 

 apprehensions of the same kind as those above referred to. 



* This Journal, 1884, pp. 292-3 



t Joum. Quek, Micr. Club, iii. (1888) p. 268. $ Ibid., p. 269. § Ibid., p. 268. 



