PROCEEDINGS OF THE SOCIETY. 191 



distance between two adjoining areolations to be the 1/24,000 in., 

 and the interspaces being about equal, or the 1/48,000 in., and esti- 

 mating the small marking at even 1/3 of the interspaces, that would 

 give it a diameter of the 1/144,000 in. This was a thing that could 

 not, anyhow, be resolved under the oblique light system, but could 

 only be seen when the objective was full of light. 



Mr. Crisp said that Mr. Nelson was such a well-known expert in 

 such matters, that it was, perhaps, a little presumptuous for him to 

 point out that he had mixed up two entirely different questions — 

 visibility and resolution — which perhaps accounted for the misunder- 

 standing to which the paper was directed. In the same way the 

 claim to have seen 1/1,000,000 in. was supposed to have disproved 

 the limit of resolution depending upon wave-length. It was, however, 

 only a question of visibility, wliereas the diifraction theory, in this aspect 

 of it, referred to resolution. In the last case, put by Mr. Nelson, so far 

 from the resolution not being effected by oblique light, it was oblique 

 light and nothing else that resolved the object. As to the notion that 

 the better the diffraction spectra were seen, the better they could see 

 an object, he (Mr. Crisp) now heard it for the first time, he did not 

 understand where Mr. Nelson could have found such a statement. 

 Again, no theory that he was aware of, suggested that there was such 

 a " reduction of the power of resolution " as Mr. Nelson had referred 

 to. No such reduction in fact took place. 



Dr. Matthews said he was one of those who had been stumbling 

 over this question, and it had seemed to him that resolution and 

 visibility meant very much the same thing. Most photographers 

 would bear out the statement, that a picture taken at midday was 

 never so effective as one taken when the sun's rays fell upon the 

 objects at a greater angle, and when the contrasts of shadows enabled 

 the eye to perceive the details in a more effectual manner. Just in 

 this way it seemed to him that when a thing was said to be "resolved," 

 it meant that its component parts seemed to be more visible. 



Mr. Crisp said, the difference between visibility and resolution 

 would be understood from the fact that though they might be able with 

 a dry objective to see a line which measured the 1/1,000,000 in., 

 yet they could not separate two or more of such lines. In reference 

 to a question from Mr. C. Beck, he further said that there could be no 

 manner of doubt as to the difference made by Prof. Abbe between 

 resolution and visibility, and read the following quotation from Prof. 

 Abbe's original paper : " Such objects cari be seen however minute ifiey 

 may he ; this is merely a question of contrast in the distribution of 

 light, of good definition in the objective, and of sensibility of the 

 retina. In point of fact, neither Prof. Helmholtz nor the author 

 have ever spoken (as, however, has so often been supposed) of a limit 

 of ' visibility ' — only of a limit of visible ' separation.' " (Cf. Vol. I., 

 1881, pp. 415-6.) 



Mr. Nelson drew a diagi'am upon the board, showing the appear- 

 ance under the Microscope of the spicule to which he had previously 

 referred, and which he said he saw perfectly with an immersion 

 objective of 1*43 N.A. and a dry achromatic condenser. How was it 



