1882.] 



MICROSCOPICAL JOURNAL. 



131 



iting here to the appreciative assem- 

 blage which they would have around 

 them, say 75,000 lines to an inch re- 

 solved with the low apertures refer- 

 red to ! 



We have seen that on the one hand 

 the depth of vision decreases as the 

 aperture is increased, and that on the 

 other, as the objects become smaller 

 and smaller the similarity of their 

 images increases with the increase in 

 the aperture — the one representing a 

 disadvantage attendant upon large 

 aperture and the other an advantage 

 — and bearing this in mind we are in 

 a position to arrive at a correct view 

 of the relative value of objectives 

 with large and small apertures, which 

 I take to be this : — 



Both kinds of objectives are neces- 

 sary for investigations into the struc- 

 ture of minute objects, and an ob- 

 server to be fully equipped, should 

 provide himself with two objectives, 

 one of moderate and one of wide 

 aperture. The former would be used 

 for the more general survey of the 

 various parts of the object, and the 

 latter for the subsequent examination 

 of its minute structure. In searching, 

 for instance, through a stratum of 

 fluid for Bacteria, a wide aperture 

 would be unnecessary, but when a 

 particular Bacterium is found, it is 

 only that which will give us an accu- 

 rate view of its flagellum. 



But again, in the choice of the ob- 

 jectives, 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 ob- 

 jects for which such powers would be 

 used is not sufficiently minute to re- 

 quire large apertures for their proper 

 delineation, and we therefore expose 

 ourselves to the disadvantage of very 

 restricted penetration and the trouble 

 of delicate manipulation, 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 ex- 

 amination 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 ques- 

 tions, which, after all have so impor- 

 ant a bearing on our practical work, 

 reference need only be made to the 

 descriptions published in our Journal 

 of new inventions in regard to me- 

 chanical and optical appliances (most 

 of which have been exhibited at our 

 meetings) to prove that great pro- 

 gress 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 elabo- 

 rate accessory apparatus of all kinds, 

 indicate not only the activity of mind 

 and the abundance of the resources 

 of the microscopical optician, but 

 that these things are really required 

 in a progressive science. 



It is to be hoped that the possess- 

 ion of excellent instruments and con- 

 venient apparatus will incite many of 

 the Fellows to undertake more care- 

 ful 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, be- 

 fore committing one's self to any sta- 

 tement, to search and prove its origi- 

 nality. 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 



