288 



Mr. T. F. Smith read a paper " On True and False Images in Microscopy." 

 Mr. E. M. Nelson said he should just like to mention, by way of correction, 

 one little mistake which Mr. Smith had fallen into, and that was in making 

 the statement that Prof. Abbe's diffraction theory was first published in the 

 " Royal Microscopical Journal." He did not seem to be aware that it was 

 in the first instance absolutely ignored not only by that, but by all the 

 scientific journals of this country, until Dr. Fripp translated it, and got it 

 printed in the " Bristol Naturalists' Journal." There had no doubt been 

 some very objectionable passages written in connection with the subject — not, 

 perhaps, by Prof. Abb6, but in such a way as to appear to put them into 

 Prof. Abbe's mouth, such, for instance, as the statement that because the 

 whole of the diffraction images were not taken in, therefore the whole 

 structure of the object could not be known. That, of course, was absurd on 

 the face of it, and Prof. Abbe did not believe anything of the kind. The 

 diffraction theory did not disturb any of the existent laws ; it left them just 

 where they were, coming continuously on and up until a point was reached 

 where mathematical theory showed that the object was beyond the grasp of 

 the lens that was being used. If they had an object within the powers of the 

 lens, then, of course, they would be able to get a true idea of its structure, 

 but if not, then they might get any appearance, according to circumstances. 

 Touch was a very wonderful thing, enabling them to form ideas of the form 

 of objects, but they might as well say that they could know nothing about 

 objects because they could not touch them. With these difficult objects, 

 however, though they could get a fair knowledge of them within the limits 

 of their optical powers, yet they came at length to a point where the large- 

 ness of the angle required was such that they could not yet grasp the diffrac- 

 tion spectra, and at that point their entire knowledge necessarily ended. 



The President said that he had listened with great interest to the paper 

 and the remarks made upon it ; but he feared that justice had not been 

 done to Prof. Abbe's work. With regard to diffraction spectra, very few 

 people understand them, and he did not know that he could make the matter 

 very plain to those who have not studied the subject. Not only is it neces- 

 sary for the comprehension of this subject to have a competent knowledge of 

 the laws of refraction and vision — mechanical optics — but it is also necessary 

 to know something of the more complex science known as physical optics. 

 The manner of regarding light is quite different in mechanical and physical 

 optics. In the former science a ray of light is represented by a straight line, 

 that is, the direction of the propagation of the light only is taken into account. 

 In physical optics light is regarded as a series of wave-surfaces. The dis- 

 turbances taking place in planes at right angles to the rays are taken into 

 account. As these disturbances are very minute, the convention that light 

 travels in straight lines is sufficiently accurate, except when the objects seen 

 are very minute and very close together. When sound-waves pass through 

 an aperture they radiate just as they radiate from the point where the sound 

 is produced — this is because the opening is very small in proportion to the 

 waves ; with light the same thing takes place with a very small aperture. 

 Suppose, for example, a long wave at sea passing up the channel ; it remains 



