Stargazing: Past and Present

by Sir Norman Lockyer

Now we have in part turned the tables, and Mr. Chance, of Birmingham, owing to the introduction of foreign talent, has since constructed discs of glass of a workable diameter of twenty-five inches for Mr. Newall’s telescope, and for the American Government he has completed the large discs used in constructing the refractor of 26 inches’ diameter for the observatory at Washington (the Americans are never content till they go an inch beyond their rivals), while M. Feil of Paris, a descendant of the celebrated Guinand, has also made one of nearly 28 inches’ diameter for the Austrian Government.

Messrs. Chance and Feil, however, have the monopoly of this manufacture, and the production of these discs is a secret process. What we know is that the glass is prepared in pots in large quantities, it is then allowed to cool, and is broken up in order that it may be determined which portions of the glass are worth using for optical purposes. These are gathered together and fused at a red heat into a disc, and it is this disc which, after being annealed with the utmost care, forms the basis of the optician’s work.

For the glass used for reflectors, purity is of little moment, as we only require a surface to take a polish, since we look on to it, and not through it; but in the case of the glass that has to be shaped into a lens the purity is of the utmost importance. The practical and scientific optician, on his commencement to make an object-glass, will grind the two surfaces of both flint and crown as nearly parallel as possible, and polish them. In this state he can the better examine them as to veins, striæ, and other defects, which would be fatal to anything made out of it. He has next to see that the annealing is perfectly done by examining the discs with polarized light, to see by the absence of the “black cross” that there is no unequal tension. It is so difficult to run the gauntlet through all these difficulties when the aperture is considerable that refractors of forty inches’ aperture may be perhaps despaired of for years to come, though the glassmaker is willing to try his part.

Next, as to metallic specula. As we are dealing with the instruments that are now used, we will be content with considering the compounds that have been made successfully, and omit the variations which have never been brought into practice. To put it roughly, the metal used for Lord Rosse’s reflector consisted of two parts of copper and one part of tin; but here we have an idea of the Scylla and the Charybdis which are always present in these inquiries. If we use too much tin, which tends to give a surface of brilliancy to the speculum, a few drops of hot water poured on it will be enough to shiver it to atoms. This brittleness is objectionable, and what we have to do is to reduce the quantity of tin. But then comes the Charybdis. If we do this, the colour is no longer white, but it is yellow, and in addition we have introduced a surface that quickly tarnishes instead of a surface which remains bright. The proportions which seem to answer best are copper sixty-four parts and tin twenty-nine. Lord Rosse, we believe, uses 31·79 per cent, of tin; or very nearly the above proportions. Mr. Grubb in the Melbourne mirrors used copper and tin in the proportion of 32 to 14·77.

Having the metal, we have roughly to cast it in the shape of a speculum, but if an ordinary casting is made in a sand mould the speculum metal is so spongy that we can do nothing with it. If it is put in a close mould it will probably be cast very well, but it will shiver to atoms with a very slight change of temperature. The difficulty was got over by Lord Rosse, using an open mould called a “bed of hoops;” the bottom of the mould being composed of strips of iron set edgeways, held together by an iron ring and turned to the proper convexity; sand is then placed round the iron to form the edges, the metal is then poured in, and the bubbles and vapours run down through the small apertures at the bottom of the mould, so that the speculum is fairly cast. Mr. Lassell proposed a different method, which was introduced by Mr. Grubb in his arrangements for the Melbourne telescope. Instead of having the bottom of the bed of hoops perfectly horizontal it is slightly inclined; the crucible, which contains the metal of which the speculum is to be cast, is then brought up to it—the amount of metal being something under two tons in the case of the Melbourne telescope—and the bed of the mould is kept tipped up as the metal is poured into it, and so arranged as to keep the melted metal in contact with one side; and as it gets full it is brought into a perfectly horizontal position.

Having cast the speculum, the next thing is to put it in an annealing oven, raised to a temperature of 1,000°, where it is allowed to cool slowly for weeks till it has acquired nearly the ordinary temperature. On being removed from the oven the speculum is placed on several thicknesses of cloth and rough ground on front, back, and edge.

Having got the material roughly into form we now pass on to see what is done next.

In the case of the reflector, whether of metal or glass, the optician next attempts to get a perfectly spherical surface of the proper curvature for the required focus.

In the case of the refractor matters are somewhat more complicated; we have there four spherical surfaces to deal with, and the optician has work to do of quite a different kind before he even commences to grind.

Presuming the refractive and dispersive properties of the glass not known, it will be necessary to have a small bit of glass of the same kind to experiment with. That the optician may make no mistake in this important matter, some glass manufacturers make the discs with projecting pieces to be cut off; these the object-glass maker works into prisms to determine the exact refraction and dispersion, including the position in the spectrum of the Fraunhofer lines C and G, for both the crown and flint glass. With these numbers and the desired focal length he has all the necessary data for the mathematical operation of calculating the powers to be given to the two lenses—flint and crown, and the radii of curvature of the four surfaces in order that the object-glass may be aplanatic or free from aberration both spherical and chromatic. The problem is what mathematicians call an indeterminate one, as an infinite number of different curvatures is possible. Assume, however, the radius of curvature of one surface, and all the rest are limited. In assuming the radius of curvature on one of the crown-glass surfaces, it is well to avoid deep ones. It is better to divide the refraction of the four surfaces as equally as the nature of the problem will admit, as any little deviation from a true spherical figure in the polishing will produce less effect in injuring the performance of the object-glass from surfaces so arranged than if the curves were deep.