
PROFESSOR MARTIN DUNCAN’S ADDRESS. 341 
mi alin: nig a A a a 
the definition which this lens yields is absolutely unsurpassed in 
my experience.” 
The question of the power of resolution supposed to be pos- 
sessed by small apertures can also be brought to a very simple 
practical test by those who believe in that view exhibiting here to 
the appreciative assemblage which they would have around them, 
Say 75,000 lines to an inch resolved with the low apertures 
referred 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 :— 
oth kinds of objectives are necessary for investigations into the 
structure of minute objects, and an observer to be fully equipped, 
should provide himself with fwo 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 accurate view of the 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 manipu- 
lation, 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 
