2 Transactions of the Society. 



for that matter any ability, to tell you something new. It will be 

 of necessity a repetition of an oft-told story, but not I trust 

 entirely devoid of interest even to those of us to whom it is most 

 familiar. There is in this Society perhaps too great a disposition ta 

 avoid what is elementary. I can hardly imagine that any of u& 

 are so well grounded in the principles of microscopy that no further 

 consideration is necessary. 



In fact the exhibits here from time to time convince me that 

 while the objects are often of great interest, the methods of showing 

 them do sometimes leave much to be desired. That will only be 

 remedied by an educational effort, so that all of us may have the 

 opportunity of understanding the scientific principles of the subject 

 we are interested in, and of applying those principles in all we 

 undertake. This, then, is my excuse for dealing with the elements 

 of the subject at this time. It may be only an excuse, and not a 

 well-founded argument, but I can assure you that it has at least 

 the merit of being honest in intention. 



I need hardly remind you that I shall be unable, with the time 

 available, to do more than glance here and there at general prin- 

 ciples, touching on and indicating points of interest, but not dealing 

 with any part of my subject in a comprehensive or exhaustive 

 manner. 



In any consideration of the limitations imposed by the micro- 

 scope, a clear conception of the magnitudes involved and the relation 

 of microscopic resolution to wave-lengths of light and to molecular 

 dimensions is necessary. This point is of importance, because 

 while the limits of resolution are deiinite, and unfortunately soon 

 attained, the limits of visibility are by no means so inflexible, and 

 lead us to a consideration of bodies that may approach an organic 

 molecule in order of size. 



As you are all aware, the unit of measurement which is usually 

 adopted in microscopic work is the micron, represented by the 

 Greek sign fi. In observations that involve a consideration of 

 dimensions comparable to wave-lengths of light it is more con- 

 venient to adopt the micro-millimetre, which is one -thousandth of 

 a micron, and which therefore bears a convenient relationship to 

 dimensions that are of a much smaller order of size. I know of no 

 more graphic method of demonstrating the relative size of these 

 various dimensions than that adopted by Sir George Beilby when 

 dealing with the thickness of thin metal films, and the diagram I 

 am showing you, and the description I give, are taken from his 

 monograph on the " Surface Structure of Solids." 



Beginning on the left there is a scale in micro-millimetres. 

 Next to this are three bars which represent on the same scale tlie 

 wave-lengths of mean red, mean green and mean violet light 

 respectively. The centre of the diagram is occupied by a series 

 of seven sections, all on the same scale, of leaves or films of 



