110 
THE YOUNG SCIENTIST. 
quantity should never be sufficient to 
loain tlie eye. Few objects are seen to the 
best advantage with a la7^ge pencil of 
perfectly direct light, and the beginner 
should practice till he is able to obtain 
that arrangement which produces the 
best effect. 
It is advisable that the hole in the stage 
of the microscope should be large— at least 
an inch and a half in diameter— so that 
the entrance of oblique rays is not ob- 
structed, and it is desirable that the mir- 
ror, in addition to sliding up and down, 
should have an arm by which it may be 
swung to one side. This enables such 
oblique rays to be employed as to give a 
dark field, all the light which reaches the 
eye being refracted by the object through 
which it is sent. The opticians sell 
special pieces of apparatus for this pur- 
pose, but though they are very useful, 
they do not render it less desirable to 
have the mirror mounted as described. 
Most microscopes are furnished with a 
revolving diaphragm, with three holes, of 
different sizes, to diminish the quantity of 
light that is admitted to the object. This 
instrument is of some use, and offers a 
ready means of obtaining a very soft 
agreeable light for transparent objects, 
viewed with low powers. For this pur- 
pose cut a circular disc of India or tissue 
paper, rather larger than the biggest 
aperture; scrape a few little pieces of 
spermaceti, and place them upon it, then 
put the whole on a piece of writing paper, 
and hold it a few inches above the flame 
of a candle, moving it gently. If this is 
dexterously done, the spermaceti will be 
melted without singeing the paper, and 
when it is cold the disk will be found 
transparent. Place it over the hole in 
the diaphragm, send the light through it, 
and the result will be a very soft agreea- 
ble effect, well suited for many purposes, 
such as viewing sections of wood, insects 
mounted whole, after being rendered 
transparent, many small water creatures, 
etc. Another mode of accomplishing this 
purpose is to place a similarly prepared 
disk of paper on the flat side of a bull's- 
eye lens, and transmit the light ot a lamp 
through It. This plan may be used with 
higher powers, and the white opaque light 
it gives may be directed at any angle by 
means of the mirror beneath the stage. 
An ordinary lamp may be made to 
answer for microscopic use, but one of 
the small hand lamps, now so common, 
is singularly convenient. It is high 
enough for many purposes, and can easily 
be raised by one or more blocks. A 
kerosene lamp on a sliding stand is still 
more handy, and all the better for a hole 
with a glass stopper, through which it may 
be filled. 
Many people fancy that the eyes are 
injured by continual use of the micro- 
scope, but this is far from being the case 
if reasonable precautions are taken. The 
instrument should be inclined at a proper 
angle, all excess of light avoided, and the 
object brought into focus before it is 
steadily looked at. Most people solemnly 
shut one eye before commencing a micro- 
scopic examination ; this is a practical 
and physiological mistake. Nature meant 
both eyes to be open, and usually resents 
a prolonged violation of her intentions in 
this matter. It requires but a little prac- 
tice to keep both eyes open, and only pay 
attention to what is seen by that devoted 
to the microscope. The acquisition of 
this habit is facilitated, and other advan- 
tages gained, by a screen to keep out ex- 
traneous light. For this purpose take a 
piece of thin cardboard about nine inches 
square, and cut a round hole in it, just 
big enough to admit the tube of the mi- 
croscope, about two inches from the bot- 
tom, and equidistant from the two sides. 
Next cut off the two upper corners of the 
cardboard, and give them a pleasant- 
looking curve. Then cover the cardboard 
with black velvet, the commonest, which 
is not glossy, answers best, and your 
screen is made. Put the hole over the 
tube of the microscope, and let the screen 
rest on the little ledge or rim which forms 
an ornamental finish to most instruments. 
A piece of cork may be gummed at the 
back of the screen, so as to tilt it a little, 
and diminish its chance of coming into 
contact with that important organ the 
nose. This little contrivance adds to the 
clearness and brilliancy of objects, and is 
a great accommodation to the eyes. 
One more oculistic memorandum, and 
