812 SUMMARY OF CURRENT RESEARCHES RELATING TO 



In using the words quoted by Mr. Lewis Wright, ' Only the centre 

 of the pencil falling on the back surface of the lens reaches the eye,' I 

 was speaking of the intensity of the light illuminating each portion of 

 the object, and all I meant was that the effective emergent pencil which 

 enters the pupil is small as compared with the angular aperture of the 

 object-glass. 



I admit that the sentence is ambiguous, and should have been more 

 clearly worded ; but I never intended to convey the idea that the back 

 face might be silvered, leaving only a small aperture. 



The idea of silvering the lens, as Mr. Lewis Wright suggests, did 

 cross my mind w-hen I was working at the subject, but I saw at once 

 that it could not be used as a means of settling the question, and for 

 this reason. The object-glass is made experimentally, and the outer 

 zone is of the utmost importance, as it is far more easily corrected to 

 give'a sharj) image than any other part of the lens. 



It will be readily seen that if the lens surface be divided into a 

 number of concentric zonular elements, the more nearly these approach 

 the centre of the lens, the less the angle their normals make with direct 

 incident light. 



If the aperture Avere small enough the lens would have practically 

 plane surfaces, and could not give any distinct image other than that 

 given by a pinhole. The outer zones are so corrected that the pencils 

 passing through them come to the same foci as the central jDcncils — that 

 is, their chromatic and spherical aberration is reduced to a minimum, 

 whilst the intermediate zones are left uncorrected. If by any diaphragm 

 or other appliance the outer zone is rendered ineffective, the next outer- 

 most zone must be corrected. 



I do not know whether it would be possible to reduce the apertures 

 of an objective, and re-correct the glass without increasing its working 

 distance. If this were possible, the glass might regain the definition 

 lost by the reduction of aperture, provided this reduction were not 

 great ; but the experiment would be one of great practical difficulty, and 

 could only be carried out by one of the best makers of lenses, and then 

 only witli great expenditure of time. I fear we shall have to be satisfied 

 with some less direct method of settling the question. 



In my own mind there is no doubt whatever that all definition 

 would be destroyed by silvering the back face and reducing the aperture, 

 which is practically the same thing as putting a diaphragm behind the 

 objective, whether the image is a purely dioptric or a diffraction 

 phenomenon, unless some compensatory change could be made in the 

 glasses without altering the curve of the front lens or its working 

 distance. 



I would remind microscopists that what I have said applies only 

 to critical images with high powers, and I would ask them to compare 

 such images with those seen by dark-ground illumination and lower 

 powers. The resemblance between the images produced by the two 

 methods of illumination is very striking." 



Prof. Abbe himself has also sent a paper to the Society (now in 

 process of translation) refuting Prof. Lowne's suggestions. 



Ultimate Structure of the Pleurosigma Valve. — At the October 

 meeting Mr. T. F. Smith read the following paper : — ■ 



Twelve months ago I had the honour of bringing before you some 



