The President's Address. By Prof. P. Martin Duncan. 161 



for Bacteria a wide aperture would be unnecessary, but when a 

 particular Bacterium is found, it is only that which will give us 

 an accurate view of its flagellum. 



But again, in the choice of the objectives, the proper relation 

 between magnifying power and aperture must be maintained. For 

 work with low powers, it is useless to have large apertures. The 

 structure of the objects for which such powers would be used is 

 not sufficiently minute to require large apertures for their proper 

 delineation, and we therefore expose ourselves to the disadvantage 

 of very restricted penetration and the trouble of delicate mani- 

 pulation, without any corresponding benefit. 



On the other hand, it is equally useless to work with high 

 powers (that is upon minute objects) with small apertures. We 

 should have only an empty amplification — mere increase in the 

 distance apart of the outlines, without any additional structure 

 being made visible in consequence of the defect in aperture. 



Whenever the subjects of our examination are so minute as to 

 require high amplifications in order to be seen, then we must 

 also have large apertures in order to obtain perfect delineation of 

 the objects. 



Leaving now the theoretical questions, which after all have so 

 important a bearing on our practical work, reference only need be 

 made to the descriptions published in our Journal of new inventions 

 in regard to mechanical and optical appliances (most of which 

 have been exhibited at our meetings) to prove that great progress 

 is being made in the designing, manufacture, and application of the 

 Microscope. Improved stands and eye-pieces, new immersion lenses, 

 stages, and swinging substages, more effective fine movements and 

 elaborate accessory apparatus of all kinds, indicate not only the 

 activity of mind and the abundance of the resources of the micro- 

 scopical optician, but that these things are really required in a 

 progressive science. 



It is to be hoped that the possession of excellent instruments 

 and convenient apparatus will incite many of the Fellows to under- 

 take more careful researches into the minute details of organic 

 nature, or amongst the very fascinating rocks which are being so 

 beautifully cut and mounted by petrologists. It is true that the 

 difficulty of getting upon a path of original research is very 

 deterrent. The activity of Continental and American microscopists 

 is indeed great, and it is always necessary, before committing one- 

 self to any statement, to search and prove its originality. Much 

 microscopical research is quite beyond the powers of the man who 

 has other avocations, and to whom the instrument is a pleasing, 

 and none the less important, toy. Consider the paraphernalia 

 required to study the microscopy of the details of a minute animal. 



Ser. 2.— Vol. II. M 



