The President's Address. By H. C. Sorhj, F.B.S., &c, 113 



the object would give them at such an interval and situation as 

 would obscure the structural lines in another part, and by no 

 single adjustment could the whole be seen correctly, but in all 

 cases true and spurious lines would be mixed up together. The 

 only chance of arriving at a true knowledge of the real structure 

 would be by a careful induction from the facts observed when the 

 illumination is made to vary ; and even when a satisfactory con- 

 clusion could thus be drawn it would only be by acting on the 

 principle that the limits of simple and distinct visibility had been 

 passed, when light has to be treated as an agent scarcely fitted for 

 the requirements of the case. 



When we come to the examination of single detached particles 

 the conditions are materially changed, but if the bright part of 

 the interference fringes fall on the dark boundary line of a trans- 

 parent particle or the bright part of a fringe on the centre of an 

 opaque particle, it could not be distinctly seen though its presence 

 might be recognized. 



The limit of g o^o o of an inch deduced on Helmholtz's principle 

 from the physical characters of hght agrees admirably with the 

 estimate formed independently by various great authorities on the 

 microscope. The mean of the estimate thus formed by Quekett, 

 Eoss, De la Eue, and Carpenter, as quoted by Stodder, is in fact 

 exactly the same (-so wo- o^ 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 conclusions. With an immersion object-glass of very large 

 aperture it might be possible to define an interval of somewhat less 

 than ro-^Voo^ of an inch, but probably the above-named determina- 

 tions 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 visihilitij of jnrnute objects is concerned, 

 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 -go^oo to 

 TO oWo 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 throw any light on this 

 question, and the only course as yet open to us is to draw the best 



