210 
NOTES ON OPTICAL INSTRUMENTS. 
and eye lens forming a compound microscope through which the 
apparent image is viewed. There is much more loss of light in the 
erecting eye-piece, owing to there being more lenses, than is the case 
with an inverting eye-piece. The curves of the lenses vary slightly 
with different makers. The plate gives the most modern form. 
The double Galilean telescope or binocular field glass (its usual service 
form) consists of an object glass of shorter focus than in a telescope, 
and of a double concave lens as an eye-piece. In cleaning these 
glasses care must be taken to place the surfaces of the same concavity 
towards the eye in both tubes or distortion will ensue. 
The binocular telescope simply consists of two erecting telescopes fitted 
parallel to each other, and with an adjustment to regulate the distance 
between the tubes so as to get a clear view with both eyes. Though 
very powerful, they are not to be recommended as their construction is 
very delicate, and if once knocked out of truth the eyes are strained 
by seeing two images, and they can only be repaired by an optician. 
The same remarks apply to them as to single telescopes. 
To light up cross-wires by night. —In the telescopes used with theodo¬ 
lites a ray of light is admitted through one of the trunnions which is 
bored for the purpose, and this ray is reflected by a small silver disc 
placed at an angle of 45° degrees with the optical axis of the instru¬ 
ment on to the cross-wires. 
Another way is to fix an elliptic ring (with an ellipse bored through 
it for direct vision of the telescope) with a band to the outside of the 
rim of the object glass of the telescope and to reflect the rays of a 
lamp up the tube by this means or the same may be applied in the 
shape of a cone silvered on the outside with its apex cut out and so 
reflecting the rays of any light at the side up the tube. 
In the manufacture of lenses two different sorts of glass are used, 
viz., “ crown ” and “ flint ” glass. The latter is whiter and heavier 
than the former. 
