THE YOUNG SCIENTIST. 
109 
magnified to " one hundred diameters," or 
one hundred linear ; and the figures 100 pre- 
ceded by the sign of multiplication (thus, X 
100) would be appended to any drawing 
which might be made from it. It is, how- 
ever, obvious that the length is magnified 
as well as the breadth ; and hence the mag- 
nification of the whole surface, in the 
instance specified, would be one hundred 
times one hundred, or ten thousand : and 
this is the way in which magnification is 
popularly stated. A few moments' con- 
sideration will show that the scientific 
method is that which most readily affords 
information. Any one can instantly com- 
prehend the fact of an object being made 
to look ten times its real length ; but if 
told that it is magnified a hundred times, 
he does not know what this really means, 
until he has gone through the process of 
finding the square root of a hundred, and 
learnt that a hundredfold magnification 
means a tenfold magnification of each 
superficial dimension. If told, for ex- 
ample, that a hair is magnified six hun- 
dred diameters, the knowledge is at once 
conveyed that it looks six hundred times 
as broad as it is ; but a statement that the 
same hair is magnified three hundred and 
sixty thousand times, only excites a gasp- 
ing sensation of wonder, until it is ascer- 
tained by calculation that the big figures 
only mean what the little figures express. 
In these articles the scientific plan will 
always be followed. 
If expense is not an object, a binocular 
instrument should be purchased, and it is 
well to be provided with an object-glass 
as low as three or even four inches focus, 
which will allow the whole of objects 
having the diameter of half an inch or 
more to be seen at once. Such a low 
power is exceedingly well adapted for the 
examination of living insects, or of the 
exquisite preparations of entire insects, 
which can now be had of all opticians. 
Microscopes which have a draw tube can 
be furnished with an erector, an instru- 
ment so called because it erects the 
images, which the microscope has turned 
upside down, through the crossing of the 
rays. This is very convenient for making 
dissections under the instrument ; and it 
also gives us the means of reducing the 
magnifying power of an object-glass, and 
thus obtaining a larger field. The erector 
is affixed to the end of the draw tube, and 
by pulling it out, or thrusting it in, the 
rays from the object-glass are intercepted 
at different distances, and various de- 
grees of power obtained. 
A binocular microscope is most useful 
with low powers from two-thirds upwards. 
A new form, devised by Mr. Stephenson, 
acts as an erector, and is very valuable 
for dissections. It works with high 
powers. 
Beginners will be glad to know how to 
obtain the magnifjdng power which dif- 
ferent objects require, and it may be 
stated tliat, with a full-sized microscope, 
a two-inch object-glass magnifies about 
twenty-five diameters with the lowest 
eye-piece ; a one-inch object-glass, or two 
thirds, from fifty to sixty diameters ; a 
half-inch about one hundred ; a quarter- 
inch about two hundred. The use of 
deeper eye-pieces adds very considerably 
to the power, but in proportions which 
differ with different makers. One instru- 
ment used by the writer has three eye- 
pieces, giving with a two thirds object- 
glass powers of sixty, one hundred and 
five, and one hundred and eighty respec- 
tively ; and with a fifth, two hundred and 
forty, four hundred and thirty, and seven 
hundred and twenty, which can be aug- 
mented by the use of a draw tube. 
It has been well observed that the il- 
lumination of objects is quite as impor- 
tant as the glasses that are employed, and 
the most experienced microscopists have 
never done learning in this matter. Most 
microscopes are furnished with two mir- 
rors beneath the stage, one plane and one 
concave. The first will throw a few 
parallel rays through any transparent 
object properly placed, and the latter 
causes a number of rays to converge pro- 
ducing a more powerful effect. The first 
is usually used in daylight, when the 
instrument is near a window (one with a 
north aspect, out of direct sunlight, being 
the best) ; and the second is often useful 
when the source of illumination is a 
candle or a lamp. By varying the angle 
of the mirror the light is thrown through 
the object more or less obliquely, and its 
