Now, remembering how many ounces were needed to leave one inch of solution under the surface and a quarter of an inch up the sides of the 6-1/2 in. mirror, using a vessel about 2 inches larger in diameter than the mirror, pour into a glass measure 10 drams of the silver solution, add next 10 drams of the ammonia, then 20 drams of potash; if the potash turns the mixture thick, pour it backwards and forwards into another vessel, or stir it with a strip of glass, or a glass rod, for half a minute; if the mixture does not turn clear (which it will not do if it has turned very thick on adding the potash), add cautiously, drop by drop, some of the ammonia solution, agitating it till it just clears; do not filter unless there are a great many floating particles through using unfiltered water, the floating particles cause minute black spots, which are, however, of little consequence, as their action is only the loss of so much light. Measure 20 drams of the sugar—and the mirror being ready to dip (the amount with the sugar added must be the required quantity previously ascertained), add the sugar, stir well, and immerse when the mixture begins to turn dark ink colour.
When silvered, well wash the mirror with water of the same temperature as itself (it may be safely sponged with a lump of cotton wool), and stand it on its edge on blotting-paper to dry. Drops of water standing long on it when drying will cause stains, which will not readily polish off, and indeed these should not be polished, as it is not right to polish one part more than another; so it is best to leave them, or if this is objected to, re-silver the mirror, when standing it in the sun and wind will soon dry the surface, but be careful not to let the dust settle on it.
Do not warm anything; if the weather is cold, everything may be left many hours in a warm room, but the equal temperature is of the utmost importance—the actual temperature is not of much consequence. More failures are due to unequal temperature than any other cause in the hands of the amateur. One careful and clever amateur informed me that he had failed six times, but on the seventh, by attending to the advice given above, the silvering was a perfect success, and all seemed then as easy and certain as it had before seemed uncertain and difficult.
THE FINAL ADJUSTMENT OF A NEWTONIAN REFLECTOR ON A STAR.
The adjustments being as near correctness as can be seen by the usual method of procedure—of which the fullest details are given in my little book, the telescope is ready to try on a star. If all does not appear satisfactory on first observing the star, do not disturb the adjustments for a while, but wait until the air is steadier, when perhaps it will be quite satisfactory. If not, turn it on a bright star; if the adjustments are at fault, the black shadow of the flat, when the star is out of focus, will not be central in the slightly expanded image, but will be on one side of the expanded disk. Use a power of 200 to 300.
If the black spot is near one of the sides corresponding with major axis of flat, the side screw will put it right. We will assume that the black spot is nearer the edge of flat nearest the mirror, and that the eyepiece is beyond the focus proper for the star; then unscrew the side screw. If the spot is nearer the other edge of flat, screw it up little by little, taking the hand out of the telescope each time, and see what the effect is. The movement and alteration can be watched while the hand is in the tube. If the spot is seen either the top or bottom of the flat that is in the direction of the diameter of tube, the middle screw must be slightly loosened and the flat revolved. Let the screw just bite so as to hold, and tighten it when correct. The definition of a star should now be perfect, and by daylight observe the adjustments; it may appear that the reflected circles are not concentric, but if the definition on a star is good in still air, consider the appearances correct adjustments, except that the reflection of the black spot on mirror may be made concentric. It sometimes happens that a persistent and rigid centreing of the reflections on face of flat are not the positions for best definition, perhaps from a slight optical eccentricity somewhere in the instrument.
The adjustments of a reflector are soon familiar and easy, and there is this to be said, the adjustments can be manipulated upon to any amount without the slightest injury to the instrument; there is no danger of any sort, and the instrument can be perfected in its adjustments by daylight, and this makes it both pleasant and convenient to leisurely work at, for experience sake. The instrument need not be out of doors either, unless an artificial star from a black pin’s head—which is better in strong sunlight than a thermometer bulb—is to be used instead of a star at night, if so let it be as high as possible.
Be careful not to screw up the small screws too tight, as the fine threads are liable to be spoiled and the screw made useless.
THE DURABILITY OF SILVER FILMS.
It sometimes happens that the films do not last so long as they are expected to do, that is, they sometimes lose their splendid lustre sooner than they should do. When it is remembered that though the process of silvering is an easy operation to perform, yet it is a delicate chemical one, in which good results are best obtained when several good conditions all meet together, this is not to be wondered at, but it is certain the instructions given above will reduce the chances of failure to a minimum, and imperfect results will be rare. The silver usually lasts a long time. I know silver films that have been in use for ten years.