are very rare, magnifyiug up to 5,000 or 
more. 
The difficulties of manipulation rapidly 
increase with the increase of magnifying 
power and as the higher powers are 
reached many auxiliary devices, aside 
from tbe lenses, are required, as, for in- 
stance, condevsers, in order to condense 
enough light onto a small object so that 
this light when diffused by the maguifica- 
tion shall still be strongfenough to enable 
the eye to see the magnified image. 
Any one wishing further information 
on such matters may ask any of those 
who exhibit large microscopes this even- 
ing, who will be glad to show and explain 
such accessories to those who wish to fur- 
ther understand them. 
Light is of course the medium bv which 
we see objects, whether with the eye or 
through instruments, and its rays are so 
fine, relative to the size of the object, that 
light may be considered a highly refined 
medium by which the smallest features 
are discernable. In fact the rays of light 
are popularly presumed to be so delicate 
as to preclude any material thing being 
ulore so. 
Oue can, however, easily conceive of 
how difficult it would be to see the head of 
a pin by means of ray of light which was 
an inch or a foot in diameter and when 
we urdertake to use very high powers on 
a microscope an important limitation 
arises through the fact that the objects re- 
quiring such high power are so small that 
their size closely approximates those 
qualities of the light which make it the 
mediuin of vision. 
In a simple way it may be said that the 
powers of an optical instrument depend 
upon the refraction or power of the lens 
to bend the rays of light. This refractive 
power also depends upon the wave 
length of the light, which for ordinary 
rays is about I-40,000 orI-50,000 of an inch. 
When therefore the object is comparative- 
ly small, say 1-100 of an inch, itis yet vast- 
ly larger than the wave length and its 
magnification is not complicated with 
optical difficulties When, Lowever, the 
size of the object is only about I-40,000 of 
an inch, as in the striae of various dia- 
toms it is quite as small as the avew 
length and when less than s-100,000 of an 
inch in size it is so much smaller as to be 
almost impossible to resolve It is quite 
doubtful whetber what it seen of such 
almost inconceivably small particles isa 
true image of their form. 
The above is mentioned to illustrate the 
limi‘ations involved in microscopy rather 
than with reference to the practical use of 
the instrument; for nearly all the work is 
performed under powers of 1,000 
diameters or less, while even the investi- 
~ 
gations of bacteria, of which so much 
is now heard, are conducted with powers 
rarely exceeding 1,200 diameters. 
The immense variety of uuseen life and 
form brought into vision by the micro- 
scope is appalling; for the hundreds of 
plants and animals visible to the naked 
eye thousands or even millions exist 
which are invisible. ‘he ability to in- 
vestigate them microscopically is even 
better than the opportunity to study the 
larger forms with the naked eye, 
Not only are living organisms, which 
are always of peculiar interest, ever ready 
at hand, but means have been devised for 
microscopical research into apparently 
more difficult fields. Thin sectious can 
be made of nearly anything and so 
mounted that the most delicate structure 
may be studied. Even rocks are ground 
so thin as to be transparent, allowing the 
microscope to determine their  con- 
stituents. Chemical substances are 
crystalized in thin films on glass slides so 
that their peculiar forms may be observed 
and the surface of opaque o!jects may be 
viewed by reflected light. 
We cannot now dwell on how material 
is thus prepared for investigation, but in 
the slides which are in the instruments 
this evening you will be able see many of 
these forms and further details can be 
gained from those who personally con- 
duct the several exbibits. 
It is known to comparatively few that 
light may be used in another form than 
thatin which it commonly meets the eye. 
This form 1s known as polarized. It 
would be difficult and out of place to here 
explain the difference between polarized 
and plain light. A crude analogy, how- 
ever, would be to say that common light 
has its rays and vibrations more or less 
jumbled together in an apparently com- 
plicated and unsystematic way, while in 
polarized light the rays and vibrations 
are so straightened out as to assume an 
orderly arrangement. 
Perhaps auother way of illustrating it 
by analogy would be to consider water 
falling in a stream and breaking into 
drops like rain, representing no particular 
order. This would represent plain light, 
but if the water was allowed to flow 
through several narrow slits it would first 
appear in thin sheets which when broken 
into drops would cause these Grops to be 
more symmetrically arranged than if they 
had merely fallen in aconfused mass, and 
this would typify polarized light. 
The polariscope attachment to the 
microscope, which is shown on some of 
the instruments here !o-night, so modifies 
the light as to enable certain character- 
istics of objec s to be brought out which 
are not visible under plain light. Thus 
