Special attention should be paid to the cleaning of the objective lens, which is liable to become covered with dust owing to its position in the telescope and the opening and closing of the draws.
Never on any account touch the glass with the finger or thumb. If the glass be allowed to get damp, fogging will result. To cause the fogging to evaporate, remove object-glass and eye-piece, lay the telescope out in the sun or in a warm room. Never permit the metal work to get hotter than the temperature of your hand, otherwise the Canada Balsam (which is used to join the concave and convex lenses in the object-glass of all high telescopes, except the G.S.) will melt. If the draws get wet, they must be thoroughly dried and slightly lubricated. The same applies to the sun-shade. When an officer is inspecting telescopes, he should inspect the cases also. In screwing tubes or cells into place, great care must be taken not to damage the threads. It is often as well to turn the screw the wrong way with a gentle pressure; the threads will then come into correct engagement, and a slight click may be heard.
The General Service Telescope
As has been stated above, Canada Balsam is not used between the lenses of the object-glass of the G.S. telescope. When a G.S. Telescope has been taken to pieces, the only difficulty experienced in assembling it again will be in the replacing of the lenses forming the object-glass. To do this two rules must be remembered:
1. The convex lens is always the nearest to the object, and, therefore, must be replaced first.
2. On the side of the lenses forming the object-glass an arrow-head will be found cut into the glass.
Before the lenses are put back the arrow-head must be completed, and the middle of the arrow must be allowed to slide over the barb or raised line in the cell.
RULES FOR USE:
- 1. Always extend your sun-shade (more O.P.’s have been given away by the light shining upon the object-glass of telescopes than in any other way).
- 2. Always mark your focus by scratching a circular ring on the focussing draw. (This will allow you to focus your glass correctly and quickly before putting it to your eye.)
- 3. Always pull out or push in the draws of your telescope with a clock-wise circular motion, and keep them slightly lubricated.
- 4. Always carry your telescope slung on your body. If you take it off and let it travel in a lorry or car the jolting will almost certainly ruin it.
- 5. Always use a rest when observing.
- 6. When looking into the sun, make a sun-shade nine inches or a foot long, to fit on the short sun-shade of the telescope. This will give you great assistance when the sun is over the German lines. It is a trick borrowed from the chamois-hunters of the Pyrenees.
- 7. Remember that when there is a mirage you will get better results with a low than with a high power of magnification. Conditions in France are more suitable to a magnification of under than over twenty-five. Excellent work can be done in the front line with a glass that magnifies only ten times. If the high-power eye-piece is used for any special purpose when reconnaissance is finished, it should be replaced by a low-power eye-piece.
- 8. When searching a given sector of ground or trench divide it into “fields of view” work slowly allowing each field to overlap. Never leave any suspicious-looking object without having ascertained what it is and why it is there.
- 9. Slight movement is more easily detected if you do not look straight at the object. Always look, a little left, right, high or low. Keenest vision is at the edges of the eye. This particularly applies to dusk or dawn.
- 10. When your object is found, consider:
- (a) Distance.
- (b) Shape.
- (c) Colour.
- (d) Size.
- (e) Position.
- Use each detail to check other details; for instance, if you can distinguish the state cockade upon a German cap you may be certain that you are not more than two hundred yards distant.
- 11. Do not forget that good results can be obtained on clear starlight or moonlight nights, by the use of night-glasses or telescopes, especially if working in conjunction with a Lewis or Vickers Gun. Generally speaking, the bigger the object-glass and the lower the magnification the better will be the results obtainable at night.
- 12. In trench warfare a really good glass-man working from our front line by day can make a most valuable wire reconnaissance.
- 13. Remember that the conditions of visibility are constantly changing; an object which is indistinct at eleven o’clock may become quite clear at eleven-five.
- 14. Always be ready to avail yourself of natural conditions. The visibility after a rain-shower is almost always good; it shows up wire and gaps in the wire, paths, ground traversed by patrols, etc. The best season for “spotting” O.P.’s is autumn, when the leaves fall and the grass withers.
- 15. It is a good thing to disguise the whole of the telescope by use of sandbags or other material around it. Great care must be taken to ascertain that such disguise is kept free from dust or grit.