THE PRESIDENTIAL ADDRESS 7 



magnification does no good. Many people find this a hard saying, 

 but it must be remembered that a large image is not necessarily 

 a good image. We are up against the same difficulty as before. 

 A point on the object is necessarily spread out into a disc in the image, 

 due to the coarseness of structure of light itself as indicated by its 

 wave-length. I cannot go into the details, but many of you will 

 know that points on the object which are something less than half 

 a wave-length, or say a one-hundred-thousandth of an inch apart, 

 cannot be distinctly separated. This is the theoretical limit for a 

 microscope using ordinary light, and it has been practically reached. 

 The early microscopists would have thought this more than satis- 

 factory ; but the limit puts a serious obstacle in the way of biological 

 and medical progress to-day. For example, the pathogenic bacteria 

 in many cases are about this size or less ; and there is special interest 

 in considering in what directions we may hope to go further. 



Since microscopic resolution depends on having a fine structure 

 in the light itself, something, though not perhaps very much, may 

 be gained by the use of ultra-violet light instead of visible light. 

 It then becomes necessary to work by photography. We are nearing 

 the region of the spectrum where almost everything is opaque. In 

 the visual region nearly every organic structure is transparent and 

 to get contrast stains have to be used which colour one part more 

 deeply than the other. In the ultra-violet, on the other hand, we 

 get contrast without staining and, as Mr. J. W. Barnard has shown, 

 the advantage lies as much in this as in the increased resolving power. 

 For example, using the strong ultra-violet line of the mercury 

 vapour lamp, which has about half the wave-length of green light, 

 he finds that a virus contained within a cell shows up as a highly 

 absorptive body in contrast with the less absorptive elements of the 

 cell. So that ultra-violet microscopy offers some hope of progress 

 in connection with this fundamental problem of the nature of 

 viruses. 



With ultra-violet microscopy we have gone as far as we can 

 in using short waves with ordinary lenses made of matter, for the 

 available kinds of matter are useless for shorter waves than these, and 

 it might well seem that we have here come to a definite and final end. 

 Yet it is not so. There are two alternatives, which we must consider 

 separately. Paradoxical as it may seem, for certain radiations we 

 can make converging lenses out of empty space ; or alternatively 

 we can make optical observations without any lenses at all. 



The long-standing controversy which raged in the nineties of the 

 last century as to whether cathode rays consisted of waves or of 

 electrified particles was thought to have been settled in favour of 

 the latter alternative. But scientific controversies, however acutely 

 they may rage for a time, are apt, like industrial disputes, to end in 



