564 PROCEEDINGS OF THE SOCIETY. 



subject being illustrated by the exhibition of the objects and apparatus 

 described, specially brought over to England for the purpose, and 

 illuminated by three arc lights arranged upon tables in the room. 

 Further illustration was also given as the paper proceeded, by means of 

 drawings on the blackboard as occasion required. 



The Chairman was sure that the Fellows of the Society had listened 

 with the deepest interest to Dr. Siedentopf 's exposition of the method by 

 which he had succeeded in making minute objects visible which the 

 highest powers of the Microscope were incapable of disclosing. It was 

 quite impossible for the mind to realise the dimensions of bodies 

 whose size was expressed in such extraordinary figures. He looked 

 forward to important results being obtained from the investigations 

 by Dr. Siedentopf's methods of the structure of the cell-walls of 

 animals as well as plants. He regarded this as the most important 

 paper he had at any time listened to in that room. 



Dr. Hebb then read the following communication from Dr. Gr. John- 

 stone Stoney, F.R.S. 



" If it had been in my power to attend the Meeting, and if I had 

 been asked to speak, I should have wished to say that no particles dis- 

 seminated through glass can be too small to be seen in the Microscope, 

 provided that, however small, they fulfil the two following conditions : — 



(1) That the light each particle emits is of sufficient intensity to 

 make it visible. 



(2) That the particles are distant from one another by intervals 

 that are not ultra-microscopic. 



With an immersion objective of N.A. 1 • 35 and an immersion con- 

 denser of 1 • 30, both carefully adjusted, and with useless light excluded 

 by a suitable stop, I have found that the practical limit of proximity 

 at which particles in a row may be, consistently with our seeing them 

 as separate objects, with indigo-coloured light of about 0*45 fi wave- 

 length, is somewhere about 0*20 or 0*19 /*— say about the -nroWo of 

 an inch. This would correspond to seeing a single pair of such objects 

 as separate if the interval from centre to centre is about five-sixths of 

 the above — say about 0*17 fi. 



Although a pair of objects can be seen as two, when somewhat 

 closer to one another than the intervals at which a row of such objects 

 must be spaced in order to make it possible to resolve them, it is a 

 curious circumstance, and one which perhaps has not hitherto been 

 taken notice of, that the above pair of objects will, in the Microscope, 

 appear to be somewhat farther asunder than they really are. 



This, which the present writer ascertained several years ago theore- 

 tically, can now be beautifully exhibited by the exquisite rulings which 

 Mr. Grayson has succeeded in producing." 



Prof. J. D. Everett desired to call attention to the close connection 

 between Lord Eayleigh's paper and Mr. Siedentopf's experiment. Lord 

 Rayleigh pointed out that no limit could be laid down to the smallness 

 of objects which could be rendered visible, although two objects could 

 not be seen as separate if the distance between them was much less 

 than half a wave-length. Mr. Siedentopf's experiment showed that an 

 object with a diameter only a small fraction of half a wave-length 



