THE YOUNG SCIENTIST. 
27 
The large hole allows light to pass from the 
mirror for the illumination of transparent ob- 
jects. In examining opaque objects, it is well 
to place a black card beneath them. 
How to Measure the Magnifying Power 
of Microscopes and Telescopes. 
There is probably no class of observations in 
which the senses are so likely to be deceived 
as in estimating the magnifying power of mi- 
croscopes and telescopes. It is not uncommon 
to find lenses of moderate powers condemned 
on the one hand as greatly less powerful than 
they really are, and on the other hand extolled 
as something wonderful. Thus we see in our 
streets cheap microscopes of very moderate 
power advertised as the most extraordinary in- 
struments, because they show the eels in paste, 
while just as often it happens that really good 
microscopes, of much higher power, are con- 
demned as weak, because, having been applied 
to the examination of objects with which the 
observer is familiar, they do not reveal any 
very extraordinary features. 
The real facts in these two cases are that the 
eels in paste, provided they are well illumin- 
ated, are quite visible to eyes of ordinary 
keenness of vision, while ordinary objects, if 
carefully studied under low powers, reveal 
many features which are not to be seen at a 
casual glance, or even by the most careful 
study by the naked eye. 
The fact that when our senses are not guided 
by accurate means for measurement they are 
very unreliable guides, will, nine times out of 
ten, be shown by the following experiment: 
Take a common opera or spy-glass, magnifying 
five to fifteen times, and allow some of your 
friends, who are unaccustomed to astronomical 
observations, to look through it at the full 
moon. In all probability they will declare that 
the glass does not magnify at all, but that, in 
fact, while the moon appears a little brighter 
and clearer, it seems rather smaller than other- 
wise! After having made this examination 
thoroughly with one eye, the other having been 
kept closed, let the person who has been ob- 
serving now look at the moon through the 
telescope with one eye, while the other is kept 
open and also directed to the moon. The obvi- 
ous difference in size will be perfectly astonish- 
ing. The mc'on seen through the telescope will 
now appear magnified to its full extent, and the 
fallacy of the first observation will be apparent. 
This simple experiment, in which the obser- 
vation is made by what is known as dovhle 
vision, one eye being aided by the instrument, 
and the other eye observing the object without 
any such aid, is the first step towards an ac- 
curate measurement of the magnifying power 
of any glass. For if we could count exactly 
the number of times that the magnified image 
is larger than that which is unmagnified, it is 
evident that we would have an exact expression 
of the magnifying power. To do this with the 
moon is not very easy, unless we have instru- 
ments which are specially adapted to the work. 
Let us therefore turn our telescope or opera 
glass to a brick wall, in which the individual 
courses are distinctly marked. The distance at 
which we should stand from it should be as 
great as possible, provided we can see and 
count the courses of bricks clearly. If we now 
look at the wall through the telescope with one 
eye, and directly with the other eye, we will 
find that each course of bricks, as seen through 
the telescope, covers several courses as seen 
without it, and it will be easy to bring the 
magnified image of one of the courses into such 
a position in relation to the unmagnified image 
that we can count exactly how many of the 
latter are covered by the former. Let us sup- 
pose that the number is five, or, in other 
words, that one brick seen through the opera 
glass covers exactly five as seen with the naked 
eye. In this case the magnifying power of the 
opera glass is obviously five diameters. 
By this simple method it is easy to deter- 
mine the magnifying power of common tele- 
scopes and opera glasses with sufficient accuracy 
for all ordinary purposes, but when we come to 
apply this system to microscopes, the condi- 
tions are so different that special appliances 
and methods are necessary. These we propose 
to consider in our next issue. 
Characteristics of Various Woods. 
An exchange gives some interesting items 
concerning the commercial value and proper- 
ties of the better known woods: 
Elasticity. — Ash, hickory, hazel, lancewood, 
chestnut (small), yew, snakewood. 
Elasticity and toughness.— Oak, beach, elm, 
lignumvitse, walnut, hornbeam, 
Even grain (for carving or engraving.) — Pear, 
pine, box, lime tree. 
Durability (in dry works).— Cedar, oak, pop- 
lar, yellow pine, chestnut. 
