138 Transactions of the Society. 



in some of these regions of discover), but, unfortunately, my time 

 and my strength are both limited and greatly taxed at present. 1 

 must content myself by alluding to one or two matters which have 

 come under my notice of late, and seem to me to demand the 

 attention and work of the Fellows of this Society, to whom, indeed, 

 such inquiries are especially appropriate. 



There is, in the first place, the whole question of what is called 

 the ultra-microscope. We may distinguish between indefinite 

 vision and definite vision. It has always been the object of the 

 microscopist to obtain definition: a picture with sharp presentation 

 of outline and shape corresponding to a natural reality. The ultra- 

 microscopist abandons this aim ; it is enough for him if he can 

 obtain evidence, by speck-like vague obstruction of light, that some- 

 thing capable of interrupting the path of a light-ray is present. 

 And when our aim is limited to the recognition of the existence of 

 such vague undefined particles, we have to extend very greatly the 

 limits of the size of the " ultimum visibile," as compared with the 

 limit assigned when something like an outline and the discrimina- 

 tion of shape is understood by the word "visible." Eemarkable 

 results as to the existence of particles indefinitely (by no means de- 

 finitely) visible have been obtained by using special modes of illu- 

 mination. The ordinary horizontal illumination, which we used 

 to call " dark -ground illumination,'" has been lately brought into, 

 use in Paris and elsewhere in the study of living microbes and 

 blood — with some success — and the name "ultra-microscopy " has 

 been applied to some of the results, such as brilliant illumination 

 of granules otherwise invisible. The most interesting application 

 of this horizontal illumination has been to the production of cine- 

 matographic films, showing actively moving spirilla driving their 

 way corkscrew-like through the blood, whilst the slower move- 

 ments of phagocytes are also recorded. The great difficulty in 

 producing such films arises from the fact that the intense illu- 

 mination necessary for rapid photography of the moving object is 

 paralysing and destructive to living naked protoplasm. It is more 

 rapidly fatal to amoeboid protoplasm than to the flagella of spirilla 

 and bacteria, but acts fatally upon both ; the former can rarely be 

 photographed in movement, the latter always remain active for a 

 sufficient time. The films made in Paris will, I believe, soon be 

 exhibited in London, being in the hands of the great entrepreneur 

 of cinematography, Pathe et Cie., at whose works the experimental 

 films were made. I hope very much that some of our own Fellows 

 may take take up this interesting but difficult practical problem, 

 and I believe that Dr. Spitta has already achieved satisfactory 

 results, though I have not been able to see them for myself. 



It is interesting in connection with this question of bacteria 

 and light to note that an interesting contribution to our know- 

 ledge of this subject has lately been made by Sir James Pewar. 



