240 ADJOURNED DISCUSSION IN LONDOK 



I have worked out some figures relating to a 2 mm. objective. 

 N.A. = 1.4, whicli gives perfect definition when used properly. 

 With the object displaced 1/100 mm. from its proper posi- 

 tion, I find that the marginal rays, instead of converging to the 

 paraxial image point, get farther and farther away from the axis. 

 This indicates how accurately it is necessary to focus at high magnifi- 

 cations. Therefore I would like to suggest that manufacturers of 

 apparatus for high power work and particularly for ultra-violet 

 microscopy should pay special attention to the problem of adjusting 

 the specimen accurately in relation to the objective. Particularly 

 when short wave-lengths are being used, as in ultra-violet micro- 

 scopy, is this necessary if much time is not to be wasted in taking 

 useless photographs. 



The Chairman : We now proceed to the discussion 

 of the Optics of the Miceoscopk, and I will ask 

 Mr. Whipple, President of the Optical Society, to take 

 the Chair. 



Mr. R. S. Whipple : I think that at this stage of the proceedings 

 w© ought to congratulate Messrs. Beck on the fact that they have 

 been able to produce a standard microscope and that they have 

 been able to keep their promise to produce it this month. As a 

 manufacturer I know the difficulty of keeping a promise of this kind, 

 and it is greatly to their credit that they have been able to keep 

 to time. As a manufacturer, I also know some of the difficulties 

 involved in the production of a new instrument. They have covered 

 the foot of the stand with ebonite. To do this is in itself an achieve- 

 ment; they have introduced this ingenious geometric arrangement 

 for holding the objectives, another considerable achievement. Thus 

 in this apparently simple looking article there are a number of 

 mechanical achievements — I venture to say great achievements — 

 which a few years ago would have been regarded as impossible. I 

 think, therefore, that it is not right to pass from the mechanical 

 side of the microscope without expressing our indebtedness to them 

 for what they have done so far, and to wish them and other English 

 microscope makers every success in the future. 



Abstracts of the following papers were then presented : — 

 THE OPTICS OF THE MICROSCOPE. 



Professor A. E. Conrady, " Microscopical Optics." 



Dr. H. Hartridge, M.A., "An Accurate Method of Objective 

 Testing." 



Mr. H. S. Ryland, " The Manufacture and Testing of Micro- 

 scope Objectives." 



Mr. F. Twyman, " Interferometric Methods." 



Discussion. 



Mr. Conrad Beck : I have been greatly interested in the Hart- 

 ridge test for microscope object glasses. Whether the graphs that 

 you get are any value or not, it is impossible to say. I should not 

 at present like to express the slightest opinion ; all I can say is that 

 I was interested to find that the graphs which we took in succession 



