108 THE YOUNG 
Ology and natural history, it leads to most | 
important results. 
Unfortunately, the book to which we 
have referred is somewhat scarce and 
therefore we feel that we are doing our 
readers a great service by reproducing it 
in these pages. 
PLAIN HINTS ON MICROSCOPES AND THEIR 
MANAGEMENT. 
The microscope is rapidly becoming the 
companion of every intelligent family 
that can afford its purchase, and, thanks 
to the skill of our opticians, instruments 
which can be made to answer the majority 
of purposes may be jjurchased for from 
$20 to $60, while even those of lower price 
are by no means to be despised. While 
the cheap instruments manufactured in 
quantities by those who do not even care 
to put their names on them will rarely give 
satisfaction, it will be found that the aver- 
age production of respectable houses in 
England, France, Germany, and America, 
exhibit so high a degree of excellence as 
to make comparisons invidious. We shall 
not, therefore, indulge in the praises of 
particular firms, but simply recommend 
any reader entering upon microscopic 
study to procure an achromatic instru- 
ment, if it can be afforded, and having at 
least two powers, one with a focus of an 
inch or two-thirds of an inch , and the 
other of half or a quarter. Cheap micro- 
scopes have usually onl^^ one eye-piece, 
those of a better class have two, and the 
best are furnished with three, or even 
more. 
The magnifying power of a compound 
microscope depends upon the focal length 
of the object-glass (or glass nearest the 
object), upon the length of the tube, and 
the power of the eye-piece. With regard 
to object-glasses, those of shortest focal 
length have the highest powers, and the 
longest eye-pieces have the lowest powers. 
The body of a microscope, or principal 
tube of which it is composed, is, in the 
best instruments, about nine inches long, 
and a draw tube, capable of being ex- 
tended six inches more, is frequently 
useful. From simple optical principles, 
the longer the tube the higher the power 
obtained with the same object-glass ; but 
SCIENTIST. 
a»— • 
only object-glasses of very perfect con- 
struction will bear the enlargement of 
their own imperfections, which results 
from the use of long tubes ; and conse- 
quently for cheap instruments the opti- 
cians often limit the length of the tube, to 
suit the capacity of the object-glasses 
they can afford to give for the money. 
Such microscopes may be good enough 
for the generality of purposes, but they 
do not, with glasses of given focal length, 
afford the same magnifying power as is 
done by instruments of better construc- 
tion. The best and most expensive glasses 
will not only bear long tubes, but also 
eye-pieces of high power, without any 
practical diminution of the accuracy of 
their operation, and this is a great con- 
venience in natural history investigations. 
To obtain it, however, requires such per- 
fection of workmanship as to be incom- 
patible with cheapness. An experienced 
operator will not be satisfied without hav- 
ing an object-glass at least as high as a 
quarter, that will bear a deep eye-piece, 
but beginners are seldom successful with 
a higher power than one of half-inch 
focus, or thereabouts, and before trying 
this, they should familiarize themselves 
with the use of one with an inch focus. 
It is a popular error to suppose that 
enormous magnification is always an ad- 
vantage, and that a microscope is valua- 
ble because it makes a flea look as big as 
a cat or a camel. The writer has often 
smiled at the exclamations of casual 
visitors, who have been pleased with his 
microscopic efforts to entertain them. 
" Dear me, what a wonderful instrument ; 
it must be immensely powerful;" and so 
forth. These ejaculations have often fol- 
lowed the use of a low power, and their 
authors have been astonished at receiv- 
ing the explanation that the best micro- 
scope is that which will show the most 
with the least magnification, and that 
accuracy of definition, not mere increase 
of bulk, is the great thing needful. 
Scientific men always compute the ap- 
parent enlargement of the object by one 
dimension only. Thus, supposing an 
object one hundreth of an inch square 
were magnified so as to appear one inch 
square, it would, in scientific parlance, be 
