568 PROCEEDINGS OF THE SOCIETY. 



absolute unanimity. He had been remarkably struck by the manner in 

 which Dr. Siedentopf had excluded all the light except what was 

 diffracted by the actual particles in view themselves, by the device of 

 illuminating only a thin layer of the ruby glass. The study of the 

 effects produced by particles viewed in this manner should add very 

 much to their knowledge of microscopic vision. 



Dr. C. V. Drysdale said that he had come there that evening in the 

 attitude of a searcher after truth, as he had previously seen a short 

 notice of Dr. Siedentopf's experiments indicating it should be possible 

 to see the actual molecules. As this appeared to him absolutely impos- 

 sible he was relieved to find this confirmed by Dr. Siedentopf's state- 

 ment that it was only clusters of a considerable number of molecules 

 that were rendered visible. He would remind the Members of the Society 

 that there was another limit to the vision of extremely small particles 

 besides any that might be dictated from optical considerations, viz. the 

 intensely rapid motion of the molecules themselves. Should optical 

 means be found of extending the limit of vision down to molecular 

 particles it would be necessary to reduce the temperature of the object 

 under examination to the absolute zero of temperature, and on hearing 

 of Dr. Siedentopf's work he was at first led to wonder whether he had 

 attempted something in that direction. They had been hearing a great 

 deal lately about diffraction theories, and he thought that one very 

 important point had been brought out in the paper under discussion 

 that evening, viz. that although diffraction phenomena prevented the 

 resolution jof detail in objects less in size than a half wave-length of 

 light, it did not in any way prevent the detection of an isolated particle, 

 however minute, provided its intensity of radiation was sufficient to 

 affect the retina. He thought that Dr. Siedentopf's demonstration of a 

 new optical limit of visibility was an exceedingly interesting and valuable 

 one. 



The simplest way of illustrating the visibility of small objects was 

 to consider an object like a small post fixed near the sea shore. If this 

 object was small in comparison with the size of the waves it was clear 

 that they would unite after passing it, and very little trace would be 

 left on the wave-front of the object having been encountered. It was 

 therefore obviously useless to attempt to seize such small objects by 

 transmitted light, but the waves on striking the obstacle would give rise 

 to ripples radiating from the obstacle in all directions, and it should 

 therefore be possible to see the object in any other direction almost as if 

 it had been self-luminous. Dr. Siedentopf had found it impossible to 

 get results by direct reflected light owing to the reflection from the 

 surfaces, and the great advance which he had made, and which had 

 enabled him to extend visibility so much beyond what others had done, 

 "was in his beautiful method of side illumination and especially in con- 

 centrating the light into a very narrow beam of depth comparable with 

 the focal depth of the objective. He should like to ask whether Dr. 

 Siedentopf had noticed any difference between the use of violet light as 

 compared with white light in his method. 



Finally he would remark that if Dr. Siedentopf's discovery would 

 enable us to see some of those scourges of humanity which had hitherto 



