Presidential Address. 11 



It is obvious, therefore, that the utmost care should be taken 

 to ensure accurate centration, as on this, more than on any other 

 one factor, correct image formation is dependent. 



The selection of a suitable light source is also of considerable 

 importance ; not only should the illuminant be of such a character 

 that it can be used for any purpose, but it should be so arranged 

 that the intensity can be varied quite apart from any adjustment 

 of the iris-diaphragm of the sub-stage condenser. It is often 

 possible to use a substantially larger cone of illumination if the 

 intensity of the light is suitably moderated. This is particularly 

 evident in dark-ground illumination work, in which it is possible 

 to utilize an ol)jective of considerably greater numerical aperture 

 if the illuminant is not of too great intensity. We all know what 

 is meant when critical illumination is referred to, and there is no 

 necessity for me to go over the ground again. It has been well 

 covered by others more able to deal with it than myself. But I 

 must make some protest against the differentiation between so- 

 called critical illumination and illumination which, for want of a 

 better term, I must refer to as non-critical. What is critical 

 illumination after all? Simply the application of well-known 

 optical principles in a proper manner. Nothing more. Is there any 

 other optical instrument of any complexity in the use of which 

 some special term has to be applied when the conditions are 

 fulfilled for obtaining a correct image ? Tliis is really what we do 

 in microscopy. We apply this term of critical illumination to a 

 . state of affairs where we are simply doing things properly, and we 

 are content therefore, when not using critical illumination, to put 

 up with a state of affairs that is not in accordance with optical 

 principles, and that we know perfectly well cannot give us an 

 accurate image. 



I shall be very glad to know that the term has been dropped 

 at least in this Society, not because I have anything at all to say 

 against the method or the procedure — on the contrary, I think there 

 is everything to be said for it — but we should not adopt a special 

 term to describe a process which is merely the performance of a 

 routine method in a scientific manner. 



Before I conclude may I say one word in reference to the 

 Society itself ? When you honoured me by electing me as your 

 President, whilst fully appreciating the honour, I felt that the 

 responsibility was considerable. At the time we were at the very 

 worst stage of the war, with all its discomforts and possibilities. 



I realized that the period must be one of difficulty, but I 

 think it is a tribute to this Society that we have passed through 

 it so well, and without any results that could be regarded as serious. 



The attendance at the Meetings has been quite satisfactory, 

 and the Fellowship has kept up to a remarkable degree. There 

 is one outstanding service that has been rendered to us during 



