4 Transactions of the Society. 



its constituent atoms. It will not react as they do, and on the 

 other hand it has neM^ capabilities of which they do not even 

 possess the promise or potency. In contrast with this, a drop of 

 water and hydrochloric acid mixed is simply dilute acid or acidu- 

 lated water, which you please. It will oxidise as the water does, 

 or chlorinate as the acid does, and exhibits no original properties. 

 It only reacts less actively than its components because each of 

 them attenuates the other. 



Now which of these cases illustrates the case of a wave-front 

 built up, as I say, from rays received from the condenser and other 

 rays originating in the object ? This is, so far as I know, a matter 

 which still awaits investigation, and it is one upon which, if I 

 mistake not, my photographs this evening will have a bearing. 

 But before going to the photographs, let me invite you for a 

 moment to consider the importance of tliis theory of the broad- 

 angled wave-front to the theory of the Microscope. 



Every wide-angled beam contains, of course, a narrow-angled 

 beam at its core. If by means of a diaphragm we cut off an outer 

 annulus from its margin, we leave the beam of narrower angle 

 standing. Now we know very well that the narrow cone thus cut 

 from a beam of larger angle has all the vices of the narrow beam. 

 No surviving virtue distinguishes it from the ill-bred beam which 

 never shared in the achievements of wide angles. What ought we 

 to think of its contribution to the entire result which was reached 

 when it formed the central part of a greater whole ? Did it act 

 simply as a diluent, attenuating the virtue of the annulus with 

 which it was so associated, like the water of the dilute acid, or was 

 it, like the atom of a compound molecule, an indispensable com- 

 ponent contributing according to some inscrutable law to the 

 corporate individuality of the whole ? 



Now it may turn out that the further advance of microscopy 

 will depend largely upon the answer to this question. For, if this 

 narrow-angled core is an indispensable constituent of the wide- 

 angled beam, we cannot improve our instruments by suppressing 

 it. But if, like the diluent in a weak solution, it only weakens 

 the action of the remaining constituent, then there is a chance of 

 great advance in the resolving power of our instruments, for, 

 hitherto, our widest beams have been allowed to work only under 

 the trammels of indissoluble association with beams of narrow 

 angle. The question is, can we emancipate the wide-angled cone 

 from narrow-angle tyranny, or is it the true view that both wide 

 and narrow must co-operate to produce the perfect picture ? 



Having stated the question, I will leave the answer to my 

 photographs, but still will ask leave to interpose one or two 

 remarks in explanation of the method which I have followed in 

 producing them. 



My object, as you perceive, has been to ascertain by experiment 



