106 Transactions of the Boyal Microscopical Society. 



often is not, apparent until their connection with some special 

 question is fully appreciated. It will, I am sure, be a source of 

 great satisfaction to me if what I shall say should lead to the more 

 accurate study of some of the data necessary to change my supposi- 

 tions into well-established conclusions, whether they agree with my 

 own or not. 



Though fully impressed with the imperfect state of our present 

 knowledge of the ultimate constitution of organic matter, yet even 

 now the facts are sufficiently definite to indicate, if not indeed to 

 prove, the existence of as wide a world of structure beyond the 

 limit of the power of the microscope, as what has been revealed to 

 us by it is beyond the powers of the unassisted human eye. I 

 think we may very fairly conclude that the ultimate structure, even 

 of organic bodies, will for ever be invisible, and the only chance of 

 obtaining some knowledge respecting it is by indirect methods 

 of research. For my own part, I look forward with hope and 

 confidence to a great increase in our knowledge of this question by 

 the further study of the optical characters of both organic and in- 

 organic substances, that is to say, by using light so that it may 

 sufi'er changes easily appreciated by our organs of vision, though 

 the ultimate molecules of the object examined may be so small in 

 relation to the wave-length of light, that even light itself is far too 

 coarse a means for transmitting to our eyes any distinct impression 

 of actual form or magnitude. There are also other branches of 

 physical science which serve to teach much in connection with this 

 subject, but as yet even these fail to satisfy all the requirements of 

 the case. The whole question is beset with the greatest difficulties, 

 and even when we make use of the best data hitherto obtained, we 

 see at once how very imperfect they are. One reason perhaps is 

 that the importance of the subject has not been sufficiently appre- 

 ciated, and comparatively little has been done to develop it even as 

 far as is possible. I think I may safely say that what has been 

 done relates exclusively to the elementary substances, or to the 

 most simple chemical compounds. Nothing, or next to nothing, is 

 known respecting the size and structure of the molecules of the 

 very complex substances met with in animals and plants, and when 

 we come to consider what may be their ultimate nature when form- 

 ing a part of li\dng tissue, we are immediately brought face to face 

 with questions which have probably never once attracted the atten- 

 tion of physicists ; since as a rule their studies do not lead them 

 into the consideration of biological problems. 



I propose to discuss my subject under three heads : 



1. The limit of the powers of the microscope. 



2. The size of the ultimate molecules of organic and inorganic 

 matter. 



3. Conclusions to be drawn from the general facts. 



