MICROSCOPE. 337 
for the purpose of correcting the defects inci- 
dental to it. 
In order to gain a clear notion of the mode 
in which a single lens serves to magnify minute 
objects, it is necessary to revert to the pheno- 
mena of ordinary vision. An eye free from 
any defect has a considerable power of ad- 
justing itself in such a manner as to gain a 
distinct view of objects placed at extremely 
varying distances; but the image formed upon 
the retina will of course vary in size with the 
distance of the object; and the amount of 
detail perceptible in it will follow the same 
proportion. To ordinary eyes, however, there 
is a limit within which no distinct image can 
be formed, on account of the too great diver- 
gence of the rays of the different pencils which 
then enter the eye; since the eye is usually 
adapted to receive and bring to a focus rays 
which are parallel or slightly divergent. This 
limit is variously stated at from five to ten 
inches ; we are inclined to think from our own 
observations, that the latter estimate is nearest 
the truth; that is, although a person with ordi- 
nary vision may see an object much nearer to 
his eye, he will see little if any more of its 
details, since what is gained in size will be 
lost in distinctness. Now the utility of a con- 
vex lens interposed between a near object and 
the eye consists in its reducing the divergence 
of the rays forming the several pencils which 
issue from it; so that they enter the eye ina 
State of moderate divergence, as if they had 
issued from an object beyond the nearest limit 
of distinct vision; and a well-defined picture 
is consequently formed upon the retina. But 
not only is the course of the several rays in 
Fig. 159. 
Diagram illustrating the use of the Simple Microscope. 
each pencil altered as regards the rest by this 
refracting process, but the course of the pencils 
themselves is changed, so that they enter the 
eye under an angle correspondent with that at 
which they would have arrived from a much 
larger object situated at a greater distance. 
The picture formed upon the retina, therefore, 
corresponds in all respects with one which would 
have been made by the same object, greatly 
increased in its dimensions, and viewed at the 
smallest ordinary distance of distinct vision. 
A short-sighted person, however, who can see 
objects distinctly at a distance of three or four 
inches, has the same power in his eye alone, 
by reason of its greater convexity, as that which 
the person of ordinary vision gains by the 
VOL. III. 
assistance of a convex lens which shall enable 
him to see at the same distance with equal 
distinctness. It is evident, therefore, that the 
magnifying power of a single lens, depending 
as it does upon the proportion between the 
distance at which it renders the object visible, 
and the nearest distance of unaided distinct 
vision, must be different to different eyes. It 
is ordinarily estimated, however, by finding 
how many times the focal length of the lens 
is contained in ten inches; since, in order to 
render the rays from the object nearly parallel, 
it must be placed very nearly in the focus of 
the Iens; and the picture is referred by the 
mind to an object at ten inches distance. Thus, 
if the focal length of a lens be one inch, its 
magnifying power for each dimension will be 
ten times, and consequently a hundred super- 
ficial; if its focal distance be only one-tenth 
of an inch, its magnifying power will be a 
hundred linear or ten thousand superficial. 
The use of the convex lens has the further ad- 
vantage of bringing to the eye a much greater 
amount of light than would have entered the 
pupil from the enlarged object at the ordinary 
distance, provided its own diameter be greater 
than that of the pupil. It is obviously neces- 
sary, especially when lenses of very high mag- 
nifying power are being employed, that their 
aperture should be as large as possible; since 
the light issuing from a minute object has then 
to be diffused over a large picture, and will be 
proportionally diminished in intensity. But 
the shorter the focus the less must be the dia- 
meter of the sphere of which the lens forms a 
part; and unless the aperture be proportionally 
diminished, the spherical and chromatic aber- 
rations will interfere so much with the distinct- 
ness of the picture, that the advantages which 
might be anticipated from the use of such 
lenses will be almost negatived. Nevertheless, 
the simple microscope has always been an in- 
strument of extreme value in anatomical re- 
search, owing to its freedom from those errors 
to which, as will hereafter appear, the com- 
pound microscope is subject; and the greater 
certainty of its indications is evident at once 
from the fact, that the eye of the observer 
receives the rays sent forth by the object itself, 
instead of those which. proceed from an image 
of that object. A detail of the means em- 
ployed by different individuals, for procuring 
lenses of extremely short focus, though pos- 
sessing much interest in itself, would be mis- 
placed here; since recent improvements, as 
will presently be shown, have superseded the 
necessity of all these. It may, however, be 
stated that Leeuwenhoeck, De la Torre, and 
others among the older microscopists, made 
great use of small globules procured by fusion 
of threads or particles of glass. The most im- 
portant suggestion for the improvement of the 
simple microscope composed of a single lens 
proceeded some years ago from Dr. Brewster, 
who proposed to substitute diamond, sapphire, 
garnet, and other precious stones of high re- 
fractive power, for glass, as the material of 
single lenses. A lens of much longer radius 
of curvature might thus be employed to gain 
Z 
