PROCEEDINGS OF THE SOCIETY. 441 



Mr. Hartridge regretted he had not referred in liis paper to Mr. 

 Gordon's method, but would repair the omission before publication. 



Mr. Conrad Beck said in reference to tlic letter which had just been 

 read, that Mr. Gordon's method was a well known one and frequently 

 employed with both the Microscope and the telescope. He thought 

 that indirectly, by measuring the Ramsden circle, the same result was 

 arrived at as that obtained by Mr. Hartridge, though the methods 

 were different. The Ramsden disk was a conjugate image of the aper- 

 ture of the object-glass. In one case the Ramsden disk was measured, 

 but in Mr. Hartridge's method the size of the aperture of the object- 

 glass was taken direct. Mr. Hartridge's method might have advantages 

 when it came to matters of very great accuracy, because Ramsden's disk 

 was a small aperture, and unless a fairly powerful Microscope was used, 

 errors were likely to occur. On the other hand the method to which 

 Mr. Gordon referred, and to which he thought special attention had 

 been drawm by Dr. Reid in Photomicrography, and which was described 

 in the Cantor lectures on The Theory of the Microscope, was very much 

 more easily applied than Mr. Hartridge's. In this, by taking a Micro- 

 scope with a scale, and placing it over your Microscope without touch- 

 ing it, the necessary observation could be made, while the instrument 

 was in use. If one was anxious to know^ the size of the N.A. of the 

 illumination given by the condenser, all that was necessary was to know 

 the magnifying powers of the Microscope being used, when the aperture 

 could be measured. For practical purposes where no unusual acciiracy 

 was required, such a method was better and more convenient than that 

 where the eye-piece had to be taken out, as the adjustment of the instru- 

 ment need not be disturl)ed. For more accurate purposes, however, 

 Mr. Hartridge's method might be an iinprovement. 



Mr. Rheinberg said he wished to associate himself to a considerable 

 extent with the observations of Mr. Beck. There were many ways of 

 estimating the numerical aperture or the working aperture of an objective 

 sufficient for ordinary everyday requirements. He might instance for 

 example the method of a scaled disk in the top holder at the back focus 

 of the substage condenser as brought forward by Mr. Cheshire and Mr. 

 Angus many years ago.* Mr. Gordon's method, which was familiar to 

 him, as he had made use of a similar arrangement in connexion with 

 experiments on Colour Illumination, also answered well enough for ordi- 

 nary requirements, but for any investigations necessitating the highest 

 degree of accuracy it would not compare in his opinion with the method 

 brought forward by Mr. Hartridge. 



Mr. Hartridge, in reply, said he really thought that the essential 

 difference lietween the methods used by himself and Mr. (Jordon, lay in 

 the employment by Mr. Gordon of an additional system of lenses (viz. 

 the eye-piece lenses) between the upper focal plane of the objective, and 

 the auxiliary Microscope used to project the enlarged image of Rams- 

 den's disk into the field of the micrometer eye-piece. This Mr. Hart- 

 ridge considered a disadvantage in Mr. Gordon's method ; for not only 

 would hght be lost through these additional lenses, but also the diminished 



* Journ. Quekett Micr. Club, viii. (1901) p. 211. 



