Presidential Address. 5 



and the medium. The limits of visibility are therefore mainly 

 dependent on the intensity of illumination. Using sunlight as 

 the illuminant, the smallest observable particle of colloidal gold 

 is probably of the order of 5 micro-millimetres in diameter, and 

 this may be taken to represent the smallest object that has been 

 observed. It is, however, important for us to remember that 

 colloidal gold is highly refractile, and therefore the conditions 

 obtained when it is the observed material are equalled by but few 

 other substances. It has been determined by calculation that the 

 molecules of certain albuminoids have a diameter of 6 micro- 

 millimetres. "Were these of the same optical character as colloidal 

 gold they would therefore be visible, but for the reasons I have 

 already indicated they remain invisible, and are likely to be so until 

 some new method, founded on new principles, is established. The 

 observation of gas molecules is at present much beyond the range 

 of the ultra-microscope ; but as the result of a large number of 

 microscopic observations Perrin has demonstrated that in an 

 emulsion of gamboge, the granules being uniform in size, the 

 distribution of the granules at various depths is in accordance 

 with the law connecting the density of gases with their pressure. 

 I am showing an ultra-microscope here to-night, with such 

 granules in a new type of cell which simplifies manipulation very 

 considerably. 



Such are very briefly the instrumental limitations in micro- 

 scopy at the present time, in which theory and practice, within 

 the limits of experimental error, are in agreement. In no optical 

 instrument perhaps has a nearer approach to the theoretical 

 possibilities been attained in practice. The wave theory of light 

 has been the basis of all calculations, and has proved sufficient 

 for our purpose. It has been fruitful in results, but there is no 

 finality even in this, and it is as yet impossible to say where the 

 present suggestions in physical science of a corpuscular basis to 

 account for energy transmissions may lead us. In any case it 

 would be an immense gain if the Fellows of this Society would 

 more generally follow the advances in physics, at least in their 

 elements. In this direction progress is rapid, and as it is dealing 

 with the fundamental principles governing all processes, even vital 

 ones, the necessity of a rudimentary acquaintance with such 

 principles will become more and more necessary. The recent 

 publication of such a book as that by Prof. Darcy Thompson on 

 " Growth and Form," intensely interesting in itself, is even 

 more fascinating because of the suggestions thrown out, and the 

 possibilities it opens up. If I may give a simple illustration of 

 what I mean, the usually accepted method of indicating the path 

 of rays through an optical system may suffice. In nearly all text- 

 books light is assumed to travel in straight lines, and so far as it 

 indicates the main direction of propagation in a homogeneous 



