2 Transactions of the Society, 



method was not one which would lead to any important results. 

 Very soon after this the work of Laue, Friedrich. and Knipping 

 on the use of X-rays in elucidating the structure of crystals was 

 described in the scientific press. As a consequence of this, not 

 only was the molecular structure of crystals exactly demonstrated, 

 but it was shown that X-rays were in all probability a form of 

 light, differing only in wave-length. This work was followed up by 

 Bragg in this country, and it constitutes one of the greatest advances 

 in physical knowledge that has been made in recent years. The 

 important facts that X-rays could be reflected from a crystal 

 surface, that the arrangement of molecules in certain crystals 

 could be elucidated, and the nature of X-rays themselves deter- 

 mined, have all arisen from these experiments. 



If we examine the position of Microscopy at the present time, 

 we are confronted with the fact that for the last thirty years, with 

 one exception, there has been no actual advance in microscopic 

 optics. We know perfectly well, too, that theory and practice 

 are almost in agreement : that the optician has been able to produce 

 lenses so nearly perfect that they almost reach the theoretical 

 limit. The only method by which any substantial increase in 

 numerical aperture, and consequent resolution, has been obtained 

 is by the use of ultra-violet light, the method first devised by 

 Kohler. The whole optical system is in this case of quartz, and 

 the source of light is an electric discharge between cadmium or 

 magnesium electrodes. It is an apparatus which needs consider- 

 able skill in use, and the results to be obtained by it are in some 

 respects limited, as ultra-violet light is so easily stopped by 

 organic substances. Yet it is possible by that method to make 

 use of an objective which has an effective numerical aperture 

 of 2 • 5, and resolution is increased substantially, therefore, beyond 

 the utmost that can be obtained by any other means. It has, 

 however, not really taken us very much further in practice, partly 

 owing to the difficulty of using the apparatus, and also because 

 of its high initial cost. 



The method, in fact, while of the greatest interest and utility 

 — as I have found from my own experience — is not one that 

 the microscopist may be expected to take up for general use. 



graphs of microscopic objects, using the term " microradioscopy." He further 

 indicates that he had in view the production of a grainless fluorescent screen, the 

 image on such a screen being magnified so that a direct micro-radiograph is 

 obtained. For this purpose he used different varieties of glass from the Saint 

 Gobain manufactory, some of which fluoresced under the influence of X-rays, 

 while others were opaque. The method, therefore, appears to consist of magnifi- 

 cation of the fluorescent image, the direct X-rays being stopped by the opaque 

 variety of glass referred to, which is placed between the fluorescent screen and the 

 magnifying system. It is doubtful whether such a method could succeed in prac- 

 tice, as no artificial substance, such as glass, is likely to fluoresce so brightly as 

 the mineral substances I have experimented with. 



