APPENDIX TO Hints on Silvered Glass Reflecting Telescopes.

The object of this pamphlet is to give some new and additional advice not contained in my “Hints on Silvered Glass Reflecting Telescopes,” on silvering and adjusting them; and, as the results of continued and recent experience, it is hoped they may be useful, and may prove an acceptable appendix to my little book of Hints.

The first thing to do after fastening the mirror to the wooden support, is to suspend it in the dish in which it is to be silvered, and so to adjust it that there shall be one inch between the bottom of the mirror and the bottom of the dish; then pour in water to come a quarter of an inch up the sides of the glass—the quantity thus found is to be measured, and will be the exact amount of the bath when all the solutions are mixed; this will prevent any hitch at an important moment, and the glass can be immersed without delay or disturbance.

Next proceed to wash the surface with nitric acid, taking care that the acid does not run down the sides, as it is not so easily removed from the fine ground sides as it is from the polished surface. After gently but thoroughly rubbing the surface, add a little water, and again go over; then wash all off, and take a large piece of cotton wool and well sponge the surface and sides with plenty of water, and suspend in a dish or plate with water in it.

TO PREPARE THE SOLUTIONS.

The plan I adopt is this,—I dissolve a large quantity of the chemicals required in one-fourth or one-eighth the quantity of water employed by Martin, so that in a Winchester quart stoppered bottle I can keep a large supply ready. I make the silver and ammonia solutions eight times the original strength, the potash and sugar solutions four times only, as these latter solutions require more water to properly prepare them.

Enough solutions to silver a 6-1/2 in. mirror, eight times are to be made.

Dissolve 1400 grains of nitrate of silver in 10 ounces of water (it may be clean fresh rain water filtered, if distilled is difficult to obtain, and it will act very well), but do not put the silver into the exact 10 ounces, but make up to exact 10 ounces after the silver is dissolved. It is thus condensed eight times. Do the same with the nitrate of ammonia, by dissolving 2096 grains.

Next prepare the potash and sugar solutions, condensed to four times the initial strength.

Dissolve 8 ounces of potash, and make up to exactly 20 ounces of water; if this is dissolved in a glass measure it will evolve sufficient heat to break it; it is safely done in a clean white jug. Lastly, dissolve 4 ounces of white sugar candy with 416 grains of tartaric acid, and boil ten minutes in a clean glazed vessel; when cold, add 8 ounces of alcohol, and make up to 20 ounces with water.

Do not use the solutions till all are of the same temperature, nor on the same day as they are made; and do not silver until the glass, and everything used are of the same temperature. To insure this, get the mirror ready and suspend in water, with the solutions all collected in the room in which the silvering is to be done, and let them remain until next day.

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.

Do not suppose that a long and badly worn surface will fail to show the detail and colours of the planets, the fullest detail also of the moon or the stars. It is surprising how long a silvered surface will continue to do its full work while its appearance is much deteriorated; picking up faint points of light that are near the very limit of its aperture and power, seem alone to require the perfect and fresh film. I know silver films that have been in use for seven years and have not been ever re-polished.

That the Silvered Glass Reflecting Telescope is giving satisfaction and is capable of performing the best work of a telescope—being durable, convenient, and perfectly efficient—may be gathered from the selection of a very large number of satisfactory and most gratifying letters received by the writer; and it has been most encouraging to find in all cases the observer is most eager to express his entire satisfaction, and especially so where so many have been prejudiced, in the absence of experience, with feelings of doubt as to their complete efficiency, and many, again, having used fine refractors.

It will be seen, too, that many have commenced with a small sized reflector, and gone on increasing their optical power until they have obtained a large and powerful instrument, imposing indeed as compared with what was considered a powerful instrument less than half a century ago, and at a fraction of its cost.

The gratifying success which I have achieved, attested by the universal satisfaction given by the instruments supplied by me, is the result of unwearying labour and untiring patience; every speculum, large or small, being figured with my own hands.

I have every facility for setting up instruments, and for testing, working, and regulating them in every part on celestial objects. The optical parts are, therefore, not merely tested separately and subsequently mechanically adjusted, but each and every instrument is put together and optically and mechanically tested, as a whole, before it is permitted to leave the workshop.

Every speculum is most carefully figured, and confidently guaranteed perfect to the extreme edge.

The employment of “stops” of any kind is quite unnecessary, except on bad nights, when the aperture must of course be suited to the degree of steadiness of the air. For, whether the instrument be a reflector or a refractor, the aperture and power used must of necessity be limited by the atmospheric conditions under which observations are made.

It should, however, be remembered that the reflector having not only a much larger aperture than a refractor of same focal length, and, being open to the influence of the external air, is not only affected by a relatively larger column of air, but is affected in a different way. It frequently happens that reflectors of 6 in. to 18 in. aperture, are of same focal length as 5 or 6 inch refractors.

These considerations will explain the occasional use of stops, and the expediency of having them ready to use when they can be of temporary advantage. There are nights in our climate when a 6 inch aperture may be the largest that can be used with satisfaction or advantage; indeed it used to be accepted as an incontrovertible fact that an aperture of G inches was the largest that could be used on average nights in our country. But there are also not a few nights when large apertures, bring great gain to their fortunate possessors.

It is obvious that, mutatis mutandis, these conditions must affect refractors as well as reflectors, with perhaps this difference, that in the case of a reflector the rays pass down the tube and infringe on the speculum as a column of parallel rays; consequently all the external rays of the column travel near the surface of the tube for its entire length. This being so, it is obvious that until the internal and external air and the metal tube have time to equalize in temperature, it will be advantageous, in all work requiring high powers, such as the examination of difficult double stars, to “stop off” for a time the extreme edge by a small diaphragm.

This will occur especially on frosty nights after a warm fine day. When the telescope is in the open air, ice will not infrequently form on the top of the tube, and the effect will be at once obvious in the irregularity of a star image at the edge corresponding to the top of the tube. The use of a small diaphragm will at once obviate this defect and give perfect images. On such nights a wooden tube is preferable to a metal one. It has been frequently my experience, when finally testing an instrument, that when I have been dissatisfied with its performance, and in order to trace the cause of the apparent defect, have removed the mirrors to my wooden testing tube, I have found every defect removed, and imperfect replaced by perfect images. The explanation is simple. Not only is the wooden tube much larger in diameter than the specula, but wood is of itself less sensible than metal to differences of temperature.

To make the tubes some 2 or 3 inches larger in diameter than the speculum would cure this evil, but it would involve various inconveniences and expenses in structure, which would more than outweigh the advantage of overcoming a hindrance which is after all only temporary and occasional.

The experiment of perforating or ventilating tubes has now been thoroughly and exhaustively tried, with the result that there is a fairly general consensus of experienced opinion against their use. The advantages are merely theoretical; the disadvantages are grave. Among them are the admission of cross reflections in every direction, the admission of dust, imperfect protection of the mirrors, and a decided loss of strength and rigidity to the tube.