20 On Uniformity of Nomenclature in regard to 
At what point of screw-collar adjustment the angular aperture 
and the magnifying power should he computed is one of the most 
complex questions involved in the discussion, and an entirely 
unsettled one. Most makers state the angular aperture of their 
lenses at its highest point, but no such uniformity of usage exists 
in regard to then magnifying powers. 
With the lenses of a dozen years ago this would be comparatively 
unimportant, hut with many of the high-power and high-angle 
lenses of the present day, the effect of the screw-collar movement is 
too great to be disregarded. It has been proposed, and would he 
most easy, always to rate objectives at their arrangement for un- 
covered objects, this being a naturally fixed point, and the only one 
at which the maker’s judgment in regard to the accuracy of the 
correction is usually known : but this usage would greatly under- 
rate many of the high objectives. On the other hand, rating them 
at their highest adjustment, or at an average between the two, 
might be vitiated by the fact that the point of highest correction 
is not a natural and fixed one, but is somewhat dependent on the 
judgment or caprice of the maker, some lenses of equal power 
being capable of a much larger range of corrections than others 
are. And finally, if we could agree upon some standard thickness 
of glass, and the glass were sufficiently uniform in refracting 
power, the same standard would scarcely he convenient for all 
powers (low powers being generally worked by the great majority 
of microscopists through glass, say T ^otli or T ^tli inch, for which 
many high powers are incapable of good adjustment), and few 
microscopists are sufficiently expert in the use of the screw-collar to 
make the same adjustment from the same glass-cover. Adopting 
the highest point of adjustment would perhaps involve the least 
change from present usage ; and in cases of unusual interest or 
importance it might be well to give both extremes, or else to specify 
the angle and power at which the combination was worked to 
accomplish the results specified. Attention need hardly he called 
to the fact that this great increase of power and angle, amounting 
sometimes to more than one-half of the minimum amount, is due 
entirely, not to the interposition of the cover-glass or other medium, 
hut to the change in the relation of the lenses caused by the 
movement of the screw-collar. Where an extra front of different 
properties is added, we have essentially another objective whose 
power and angle should doubtless be separately stated. 
The use of linear measurement in recording and stating powers 
has become so general that there may now be said to be no respectable 
deviation from the custom. In the early history of microscopy, 
powers were generally stated, according to the visible flatness or 
depth of the object, in superficial or cubical measure, and it was 
plausibly urged that this represented the real, visible enlargement 
