Early Criticism of the Abbe Theory. By A. E. Conrady. 647 



Abbe, moreover, dealt with a number of other points raised by 

 Altmann, and pointed out his errors ; it is interesting to note among 

 these the case of the aerial image of a grating ; here Abbe points 

 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. He, moreover, insists that when the 

 same experiment as described by Altmann is made with a white-hot, 

 and therefore really self-luminous grating, totally different results 

 are obtained. 



For this Society there is a further interest in Altmann's paper, 

 inasmuch as it obviously represents a singularly complete antici- 

 pation of a paper read before it more than twenty years later 

 by Mr. J. W. Gordon. Altmann's modified diffusion-disks are 

 completely identical with Mr. Gordon's " antipoints," and it will 

 be noted that even the arguments employed are very similar in 

 many cases. It is, of course, not even remotely suggested that 

 Mr. Gordon had plagiarised Dr. Altmann — it merely shows once 

 more how two minds may work in exactly the same way, although 

 perfectly independently. 



In conclusion, I may say that Altmann published two brief 

 attempts at a reply to Abbe's rejoinder : in the " Archiv," in which 

 the original paper appeared, the first of these replies is to be found 

 in the volume for 1 880, the second in that for 1882 ; the point 

 which Abbe had put in the forefront — the difference between a 

 self-luminous object and one illuminated from a distance — is not 

 even mentioned in either of these replies ! 



