268 Transactions of the Society. 



assume that this would be so, as otherwise there would be an 

 unnecessary loss of illuminating power. And, on the other hand^ 

 it is difficult to suppose that he did stipulate for this condition, for 

 we are told that Professor Abbe answered him by pointing out " that 

 the original grating produces diffraction spectra, and that these, as 

 well as the direct light, are refracted by the lens forming the 

 aerial image, and thus supply this latter with an exactly similar 

 set of diffraction spectra." To the case which I put forward this 

 would obviously be no answer at all — it would be sheer nonsense — 

 * for I postulated a condition of illumination which abolishes 

 diffraction by the original grating ; I showed by actual inspection 

 that the spectra were missing and demonstrated by a preliminary 

 experiment wdiat had become of them — to wit, that they had all 

 been merged in the " dioptric " image. 



Another point in which Professor Altmann's paper is said to 

 anticipate mine is that it is " an attempt " to show that " images 

 are composed of diffusion disks," formed according to Helmholtz' 

 theory of 1873. This, however, is a mere mistake. I made no 

 attempt to show anything of the kind. The fact has been matter 

 of common knowledge at least ever since the publication of 

 Sir George Airy's paper in 1835, and I took it for granted, as 

 Helmholtz himself took it for granted in 1873, and as, I suppose, 

 every other writer on the subject has taken it for granted since 

 Airy's time. Professor Altmann was too late in 1880 to anticipate 

 anybody on this point. 



There is one other point in Mr. Conrady's paper to which I 

 should like to allude for the sake of registering, with all humility, 

 my concurrence in one of Professor Abbe's positions. We are told 

 that he pointed out a certain difference as being " the real 

 distinction between his theory and that of Helmholtz." This 

 difference is stated too concisely to be intelligible to me, and I 

 therefore do not presume to express either assent or dissent as to 

 the precise definition of the difference. But I know quite well 

 what the difference is, and I entirely agree that it amounts to a 

 real distinction. In the Abbe theory no account is taken of the 

 diffraction which arises in the apertural plane of the Microscope, 

 but only of that which arises in its focal plane. In the Helmholtz 

 theory no account is taken of diffraction which arises in the focal 

 plane, but only of that which arises in the apertural plane. No 

 distinction could be more complete. The two theories have 

 nothing whatever in common. 



