Presidential Address. 7 



of the particles in depth is also controlled, to ensure that none above 

 or below the focal plane of the observing objective are brought into 

 view. 



The microscope is now a tool found in nearly all laboratories 

 devoted to scientific research, and in that sense its applications 

 are almost universal. But the ultra-microscope is a definite link 

 between the physicist and the microscopist ; the method is micro- 

 scopical, but its applications are mainly physical. The use of 

 ultra-violet light and other radiations in microscopy also demands 

 some physical knowledge. Without some acquaintance with 

 spectroscopic methods for instance, I fail to see how any worker 

 could succeed. We see therefore that in those directions in which 

 advances are probable the physical aspect is in evidence, and may 

 become predominant. Microscopy is not alone in this respect ; 

 physics threatens or promises, as we choose to regard it, to dominate 

 the whole range of scientific thought and effort. 



There is I think nothing to regret in this, rather should we 

 regard it as a natural evolutionary process. When we know that 

 the molecular structure of crystals can now be determined with 

 accuracy by means of X-rays, it may help us to realize that the 

 term " microscopy " may mean something quite different in the 

 future to what we now understand. It is simply a question of 

 keeping our minds open and in a state of flexibility, to enable us 

 to rearrange our ideas to conform to new conditions. 



The following is a description of the early experiments by 

 Tyndall, which I feel sure will be of interest, as it indicates so 

 clearly the line of thought that can be attributed to him : — 



It has been long known that light effected the decomposition of 

 a certain number of bodies. The transparent iodide of ethyl, for 

 example, becomes brown and opaque on exposure to light, through 

 the discharge of its iodine. The art of photography is founded on 

 the chemical actions of light ; so that it is well known that the 

 -effects for which the foregoing theoretic considerations would have 

 prepared us are not only probable but actual. 



But the method now to be followed, and which consists simply 

 in offering the vapours of volatile substances to the action of light, 

 enables us to give a vast extension to the operations of light, or 

 rather of radiant force, as a chemical agent. It also enables us to 

 imitate in our laboratories actions which have been hitherto per- 

 formed only in the laboratory of nature. The substances chosen 

 for examination are so constituted that when their molecules are 

 broken up by waves of light the newly-formed bodies are com- 

 paratively in volatile. To keep them in the gaseous form these 

 products of decomposition require a higher temperature than the 

 vapours from which they are derived ; hence, if the space in which 

 these new bodies are liberated be of a lower temperature than that 

 requisite to maintain the vaporous condition, they will be precipi- 



