NOTES ON OPTICAL INSTRUMENTS. 
209 
Then + 2 | = 3; and 21 -r f = 14 == magnifying power of glass. 
To adjust for collimation error (vide u Hand-book of Depression 
Range-Finder ” also). Focus the instrument on to some object exactly 
in the centre of the cross-wires. Then turn it round a half circle on 
its axis and note the amount the object moves away from the centre,, 
correct by moving the cross-wires half-way towards the object with the 
inverting eye-piece (away from the object with the erecting eye-piece) 
by the capstan-headed screws at the side,, taking care to take up as much 
one side as is given out on the other. Test again in other directions 
till the object remains in the centre in all cases. 
This error can also be eliminated by slightly inclining the object 
glass, or by turning the lenses of the object glass round in relation to 
each other. 
To fix cross-wires clean out and re-mark the old lines on the diaphragm, 
catch a spider of the small sort known as the money spider. Let him 
drop and, as he drops, catch the fresh web on a (J-shaped piece of 
wire, winding it on with plenty of space between each return of the 
thread. Apply a drop of seed-lac lacquer or brown hard varnish to 
the marks on the diaphragm, and then lay across a single thread of 
the web from the |J-shaped wire carefully and gently, and it will 
break off outside the diaphragm, the centre web remaining fixed. 
Repeat this for the other lines where necessary. 
If two telescopes get broken, a sound one may be made from the 
object glass of one and the eye-piece of another. The focal length of 
the object glass must be found and the case arranged so that the draw 
tube will bring the eye-piece down to the focal point. The convex 
lenses of an object glass may, at a pinch, also be used to replace broken 
convex glasses in the eye-piece, but care must be taken to stop out or 
blacken all except a small portion of the centre of it, and the dia¬ 
phragms and lenses may require re-adjusting. 
In ordering a new lens to replace one broken, mention its nature (see 
Figs. at. end), focal length, and diameter. If one of a doublet or triplet 
object lens the whole combination should be sent to the maker, as a 
portion of an object lens cannot be matched accurately. 
The focal length of a lens or glass can be obtained by 
measuring the distance from itself to a point which will jHHf®" 
bring the rays of the sun or a candle to a focus. Combinations JJj|f 
of lenses take their names from their form in combination. ||||| 
Thus a doublet formed of a crossed convex and a double jjUL. 
concave, as shown here, would be termed a meniscus. 
A good temporary draw tube can be made by laying a coating of 
paste or gum on a piece of fine paper and rolling it round a cylinder 
of the proper size, and this will take the screw threads of the lenses. 
A body could also be made in the same way to carry the object glass, 
but must be made strong enough and varnished over outside. 
Terrestrial and astronomical telescopes differ only in their eye¬ 
pieces, the former being erecting the latter inverting (see Figs, at end). 
The eye-piece of the former is composed of four lenses, as shown in 
the figure. The 2nd and 3rd lenses are termed condensing lenses, and 
are necessary only to bring the image into an erect position, the 2nd 
