NOTES AND MEMORANDA. 227 



-roTTTTTT^tli of an inch apart, according as a dry or an immer- 

 sion lens is used. 



The limit of g n-ro-o of an inch deduced on Helmholtz's 

 principle from the physical characters of light agrees admir- 

 ably with the estimate formed independently by various great 

 authorities on the microscope. The mean of the estimate thus 

 formed by Quekett, Moss, De la Rue, and Carpenter, as 

 quoted by Stodder, is in fact exactly the same (foo-oir of an 

 inch), so that we cannot, I think, be far from the truth, if 

 we take that as the base on which to build further con- 

 clusions. With an immersion object-glass of very large 

 aperture it might be possible to define an interval of some- 

 what less than -roiro ott of an inch, but probably the above- 

 named determinations were made with dry lenses. At all 

 events, since the limit of visibility as determined by the use 

 of the best modern microscopes agrees so completely with 

 what appears to be the limit due to the physical constitution 

 of light, we must, I think, conclude that our instruments do 

 now enable us to see intervals so small in relation to the 

 wave-length of light, that we can scarcely hope for improve- 

 ment as far as the mere visibility of minute objects is concerned y 

 whatever may remain to be done to improve their performances 

 in other respects. 



2. The Size of the Ultimate Atoms of Matter. 



Having then come to the conclusion that the limit of 

 distinct and unequivocal definition is somewhere about from 

 80-0-0-0 to To-roo-o of an inch, it appears to me very desirable 

 to consider what relation such a magnitude bears to the size 

 of the ultimate atoms of organic and inorganic matter. From 

 the very nature of the case the microscope altogether fails to 

 threw any light on this question, and the only course as yet 

 open to us is to draw the best conclusions we can from the 

 various properties of gases. This problem has been attacked 

 by Stoney,^ Thomson,^ and Clerk-Maxwell,^ who, from 

 various data, and by various methods of reasoning, have 

 endeavoured to determine the number of ultimate atoms in a 

 given volume of any permanent and perfect gas. In order to 

 avoid inconveniently long rows of figures, I have reduced all 

 their results to the number of ultimate atoms contained in a 

 space of ToVo of an inch cube, that is to say, in to o ofo ttto it 



• 'Philosophical Magazine,' 1868, vol. xxxvi, p. 132. 

 2 ' Nature,' March 31, 1870, vol. i, p. 551. 

 » Ibid., August 11, 1873, vol. viii, p. 298. 



