this will be in the fame proportion that the limits o f natural fight
bear to the focus o f the lens. If, for inftance, the convex lens is
o f one inch focus, and the natural fight of eight inches, an ob-
je ft feen through that lens will have it’s diameter apparently
increafed eight times; but as the objeft is increafed in every
direction, we mull fquare this apparent diameter, to know how
much the object is really magnified f and thus multiplying 8 by 8,
we find the fuperficies is magnified 64 times.
From thefe principles, the following general rule for afcertain-
ing the magnifying power o f Angle lenfes, is deduced. Place a
frnall thin tranfparent object on the ftage o f the microfcope,
adjuft the lens till the objeft appears perfectly diftina, then mea-
fure the diftance accurately between the lens and- the objeft,
reduce the meafure thus found to the hundredths o f an inch, and
calculate how many times this meafure is contained in eight
inches, firft reducing the eight inches into hundredths, which
will give you the number o f times the diameter o f the objea is
magnified; which number multiplied into itfelf, or fquared,
gives the apparent fuperficial magnitude o f the objeft.
As only one fide o f an objea can be Viewed at a time, it is
fufficient, in general, to know how much the furfaee thereof is
magnified: but when it is neceffary to know how many minute
objefls are contained in a larger, as for inftance, how many given
animalcula are contained in the bulk o f a grain o f fand, then we
muft cube the firft number, by which means we fha.ll obtain the
folidity or magnified bulk.
M i c r o s c o p i c a l E s s a y s , 55
The foregoing rule has been alfo applied to eftimate the magnifying
power of the compound microfcope. T o this application,
Mr. Magny, in the “ Journal d’Economie pour les mois d ’Aout
175g,” has made fcveral objeftions : one or two of thefe I fhall
juft mention : the firft is the difficulty o f afcertaining with accuracy
the precife focus o f a fmall lens; the fecond is the. want o f a
fixed or known meafure, with which to compare the focus when
afcertained. Thefe confiderations, though apparently trifling,
will be found of importance in the calculations which are relative
to deep magnifiers. T o this it may be further added, that the
fame ftandard or fixed meafure cannot be aflumed for a fhort-
fighted, that is, ufed for a well-conftituted eye. T o obviate
thefe difficulties, and fome errors in the methods which were
recommended by MefT. Baker and Needham, Mr. Magny offers
the following
P r o p o s i t i o n .
A ll convex lenfes, o f whatfoever focus, double the apparent
diameter of an objecl, provided that the objeft.is at the focus of
the glafs on one fide, and the eye is at the fame diftance, or on
the focus o f the glafs, at the oppofite fide.
E x p e r i m e n t .
Take a double convex lens, of fix or eight'inches focus, and
fix it as at A, Fig. 1, (Plate II. A,) into the piece A, which is fixed
perpendicular to the rule F G, and may be flid along it by means
o f it’s focket: the rule is divided into inches and parts. Pafte a
piece of white paper, two or three tenths o f an inch broad, and
g three