ADJOURNED DISCUSSION IN LONDON 243 



be gained. At present with apochromatic lenses the curvature of 

 the field is due to the properties of the transparent materials we 

 employ. In general they have very similar properties as regards 

 relative dispersion, and this imposes very severe limitations on what 

 can be achieved; but these limitations no longer hold if the lenses 

 are well separated, and it is possible that material improvements 

 may be effected by radical alterations in the type of objective. There 

 would be difficulties in doing this with objectives for ordinary use, 

 but they would hardly apply at all for a special instrument required 

 to give vei-y great magnification, such as the metallurgists ask for, 

 and I think these investigations might very well be made in regard 

 to objectives for this particular purpose. In fact, I think we want 

 to see a very great deal more of the design and manufacture of 

 objectives for special purposes instead of expecting one objective of 

 a given focal length to do any and every job. It ought to be realised 

 more generally that an objective of high resolving power difiEers 

 markedly from a so-called universal objective like a photographic 

 anastigmat. A microscope objective of large N.A. is necessarily a 

 very poor instrument for any conditions but precisely those for 

 which it is designed. There are many other points to which atten- 

 tion might be called, but it must suffice now to mention one. A 

 great deal has been said about the variation in the definition given 

 by similar objectives made by the same firm from similar glass, w^hich 

 ought therefore to be identical in performance, I want to suggest 

 that a possible contributory cause may be insufficiently accurate 

 centering of the surfaces. I do not think that investigations have 

 ever been carried out on methods of getting surfaces centered to an 

 extraordinary degree of accuracy, yet a veiy high degree of accuracy 

 is obviously required in a microscope objective. I have seen photo- 

 graphic lenses under examination with the interferometer, and thes9 

 have shown marked irregularities in the wave front towards the 

 periphery of the lens. When we seek the highest possible resolving 

 power, it is the periphery of the lens that is all important, so I 

 think we want to see, among other things, an investigation into 

 methods of getting surfaces centered, not twice as accurately as we 

 do them at present, but perhaps 10 or even 100 times as well. If 

 any manufacturer were able to effect such an improvement, he would 

 probably find that his lenses would realise a much more uniform 

 standard of excellence than those produced at the present time. I 

 very much hope that in some of the directions I have indicated the 

 National Physical Laboratory may be able to give assistance to our 

 own manufacturers. 



Mr. J. E. Barnard: Mr. Smith has just referred to the question 



of investigation by the use of radiations of short wave-length. I 

 should have hesitated to bring the subject up again had it not been 

 that Professor Conrady also referred to it in his paper, and by a 

 curious chance he has dropped into a not unusual error. He says 

 that the limitations of the work are in part laid down by the opacity 

 of bodies to ultra-violet light. When you get down to the dimen- 

 sions with which we are dealing in a microscopic object which is at 

 or beyond the ordinary resolution limits, opacity is almost non- 

 existent. Sir George Beilby has shown that veiy thin metal films 



