UNIFORMITY OF NOMENCLATURE IN OBJECTIVES. 141 
microscopes have a body short enough to bring the conjugate foci 
within ten inches of each other-with very low objectives, and, if 
they did, the magnifying power, instead of being that generally 
used, would be greatly reduced or altogether suppressed. 
The very low power objectives (say four and five-inch) are usu- 
ally mounted short in order to leave sufficient room between them 
and the stage,.and their power as ascertained by an arbitrary rule, 
would be greater than that at which they are usually worked, un- 
less, in their ordinary use, the draw tube were habitually raised 
enough to compensate for the shortness of their mounting. 
At what point of screw-collar adjustment the angular aperture 
and the magnifying power should be computed, is one of the most 
complex questions involved in the discussion, and an entirely un- 
settled one. Most makers state the angular aperture of their 
lenses at its highest point, but no such uniformity of usage exists 
in regard to their magnifying powers. 
With the lenses of a dozen years ago this would be comparatively 
unimportant, but 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 
be most easy, always to rate objectives at their arrangement for 
uncovered objects, this being a naturally fixed point, and the only 
one at whichthe maker’s judgment in regard to the accuracy of the 
correction is usually known: but this usage would greatly under- 
tate 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 be convenient for all 
Powers (low powers being generally worked by the great majority 
of microscopists through glass, say rto or ył inch, for which 
many high powers are incapable of good adjustment), and few mi- 
eroscopists 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 
