208 
NOTBS ON OPTICAL INSTRUMENTS. 
the other, when, if correct, they will stick together by cohesion when 
lifted gently. 
The more convex side is generally placed outwards, i.e. s away from 
the eye and the concave or flint glass next, and in a triplet lens the 
less convex surface is usually nearer the eye. 
Triplet. Doublet. Doublet. 
1. 2. 3. 1. 5. 1. 4. 
Thus1. 
2 . 
3 . 
4. 
5. 
Crossed convex. 
Double concave. 
Convexo plane. 
Concavo plane. 
Concavo convex. 
To ascertain which is the more convex side of a lens. u Surface ” 
the lens by inclining it so that a reflection is obtained off: the upper 
surface, then invert and compare the sizes of the reflections. The 
smaller the reflection the greater the convexity. 
Diaphragms are adjusted thus :— 
Push in the diaphragm with the largest opening until the eye, when 
placed in the focus of the object glass, can just take in the whole of 
the object glass and no more. Then place in the second until the 
opening in the first is a little more than free, and no reflection from 
the inside of the tube enters the eye. 
The diaphragms in the eye-piece should be pushed up the tube until 
they are, just in the focus of the lens in front of them nearest the eye. 
If cross-wires are required they are always placed across the 
diaphragm nearest the eye lens; that is in the combined foci of the 
object glass and eye-piece. 
To test a telescope for colour, etc. Direct it on to an object, such as 
a chimney pot against a bright background of sky, if there is colour 
about the edges the glass is not achromatic and should be rejected. 
Then direct it on to some bright spot against a dark background, such 
as a star at night, and adjust till it is clearly seen in the centre of 
the field of the telescope, then, without altering the focus, bring the 
spot to the extreme edges of the telescope, and if still clear and distinct 
the instrument is a good one. 
To determine the power of a telescope .—Direct it on to a brick wall, 
about 30 yards off, and focus it, then compare the number of bricks 
seen through it by one eye with the number covering the same space 
with the other naked eye. Thus, if five bricks seen with the naked eye 
just cover one seen through the telescope, the power is five to one. 
The power of an eye-piece may be considerably increased by unscrew¬ 
ing and removing the “ field-lens,” i.e. 9 that farthest from the eye, 
but the centre of the field only will be distinct. 
Another way with an inverting eye-piece is to add the focal lengths 
of the two lenses of the eye-piece , divide by two, and divide the focal 
length of the object glass by this result. 
e.g ., 2 lenses focal lengths = \ inch and inches. 
„ object glass focal length =21 inches. 
