SPECULUM. 



SPECULUM. 



704 



refer for a multitude of minute detail* to HoltzapfMV Mechanical Man! 

 puUtioo,' tu papers in the proceeding* of the British Association, 1843, 

 1850. to the ' Transaction* of the Koyal Aatronomical Society,' to the 

 ' Philosophical TranaactioDa ' for 1850, and alao to the article Speculum 

 in N ichor* ' Cyclopedia of the Phjiical Science*,' 1860. All we can 

 do, in thi* place, i* to indicate the nature of the problem to be 

 olved. and briefly to state *ome of the mean* adopted for it* solution. 



And first as to the material Speculum metal is an alloy of tin and 

 copper, which according to Lord Rowe should be a definite atomic 

 compound, namely, four equivalent* of copper to one of tin, or 

 126-4 part* of copper to 58*9 of tin, or S3 to 15 nearly. This is the 

 alloy uaed by Newton in the first reflecting telescope. It i* very 

 brilliant, but so brittle a* to break with a slight blow or sudden change 

 of temperature, and so hard and friable that it cannot be worked with 

 ateel tool*. The introduction of other metals or a larger proportion of 

 copper may diminish the brittleneu. but will lower the brilliancy, and 

 produce a metal more liable to tarnish. It is remarkable that speculum 

 metal displays it* porosity with the aid of a simple Coddington's 

 microscope. [BBOSZi: ; SI'KITU M METAL.] 



When it U stated that the 6 feet speculum of Lord Uoeae's telescope 

 weigh* 4 tons, it may be imagined that the casting of such a mass i* a 

 work of some difficulty. If a close mould were used, or if the metal 

 were rapidly cooled, it would fly to pieces in attempting to work it, or 

 if cast in sand, as an open casting, it would probably have a spongy or 

 crystalline texture, which would be visible when polished. Small 

 specula are cut in sand, and as soon a* they are set the sand core is 

 pushed out of the central aperture of such as are intended for Grego- 

 rian*, and the red-hot disc is surrounded by ignited wood ashes to 

 delay the cooling ; but in large specula the margins solidify first and 

 from want, of ductility the central part* are torn away in the act 

 of contracting, and the mass becomes rent or flawed. To prevent 

 these defects and accidents, the speculum is cast on a chill or surface 

 of iron moderately heated, the effect of which is to give a fine grain 

 and increased compactness to the metal to a small distance from the 

 surface. In Mr. Laasell's mode a cast iron mould a little deeper than 

 the speculum, with it* bottom convex, and of the same radius, is 

 attached to the end of a strong weighted lever, so that when the 

 mould i* empty, it* bottom makes a considerable angle with the 

 horizon, but become* horizontal when charged with the proper 

 quantity of speculum metal. The fused metal is poured into a lateral 

 cell, communicating with the mould at it* lowest point. In this way 

 the metal rises smoothly and evenly along the bottom, and entangled 

 air and scoria are got rid of. In Lord House's arrangement the bottom 

 of the mould i* made of pieces of hoop iron on edge, wedged tightly in 

 an iron frame, and turned to the proper curvature. This holds the 

 fused metal, but allows the gaseous matter to pass freely through. On 

 this a wooden pattern is laid about twice as deep as the intended 

 speculum and 1-6 5th larger to allow for contraction : sand is rammed 

 round this, and when the pattern is removed and the metal poured 

 into the cavity, the lower surface is chilled, the side* in contact with 

 the sand next harden, while the central part* remain longer fluid, and 

 the top or back of the speculum congeals last, by which arrangement 

 the contraction and irregularity of texture occur where they are least 

 injurious. While the cast is still red-hot, in which state it is not 

 brittle, it is transferred to an annealing furnace, which has been kept 

 beatod for some days so that it* interior brickwork is at a full red, and 

 here it is left to cool for a month or six week* in case of a 6-feet 

 speculum ; smaller ones, of course, requiring less time. 



The grinding and the polishing, so as to produce a brilliant polish 

 and a true parabolic figure, are still more difficult than the casting. 

 The grinding is done by means of a tool on which are cemented small 

 piece* of gritstone, dressed to the convexity of the surface by means 

 of a gauge of sheet-iron. A convex and a concave gauge should be at 

 hand : circular arcs are struck on them of a radius equal to twice the 

 focal length, and afterwards left free by filing and grinding. When an 

 even surface is produced by this tool, another is taken, of cast-iron, 

 turned to the convex gauge, and cut up into small squares by two sets 

 of groove*, about a quarter of an inch wide. This is charged first with 

 Band, then with emery and water, and is made to traverse the face of 

 the speculum by machinery. There must be occasional washing, and 

 emery of increasing fineness used, till all scratches are removed. The 

 face may be tested optically from time to time, and examined as to the 

 focus. Early in the process the edge of the speculum i* ground true 

 wiy sand applied l>y means of a divided hoop of iron, which admits of 

 bang tightened. The back may also be made uniform. In the grind- 

 ing, care was taken to prevent the slightest jar of the grinder on the 

 speculum, as the metal i* so brittle that there is great danger of 

 breaking it. A number of thin wooden wedges may be placed on 

 the margin of the speculum, and the polisher slowly lowered upon 

 them, and then by degrees they can be gently withdrawn. 



In the polishing process, the tool, sometimes of lighter material than 

 the grinder, U coated with pitch to a small thickness, but not closing 

 the groove* which divide the surface into squares. The pitch must be 

 of the degree of hardness such that a sovereign will stand on it on 

 edge for one minute, and leave the impression of four nicks of the 

 milling. The speculum is cleaned from emery or dust, and properly 

 secured on the polUhing-machine, and is smeared with a mixture of 

 water and rouge (peroxide of iron), and the polisher, at the temperature 



of 80*, i* placed on it for a short time. On raising it, all the square* 

 of the pitch must have been in full contact with. the speculum. If 

 this be the case, the polisher is ready for work. In the old method of 

 polishing by hand, the polisher i* fixed to a firm block, and the ope- 

 rator, holding the speculum by a handle cemented to it* back, works it 

 backwards and forward* across the polisher by straight strokes, after a 

 few of which he shift* hi* position to give them a new direction, 

 turning the speculum somewhat to get rid a* far a* may be of 

 inequalities in the abrasion ; and from time to time these cross-strokes 

 are varied by circular ones; and as the moisture evaporates, a little 

 water must bo supplied at the edge. The adhesion nn<l friction 

 increase rapidly ; the poliahing-powder changes in colour, and at last 

 disappears, when a fine polish covers the speculum, and the operator, 

 by a few circular strokes, completes the parabolic figure. 



This may be tested by optical mean*. Care must be taken not to 

 pass the proper figure, for it cannot be recovered except by re-grinding 

 and repeating the whole process. The advocates for thi* laborious 

 kind of work attach much importance to the touch of the operator, 

 since he can ftel if anything U going wrong, and correct it in time. 

 But as hand-polishing cannot be applied to specula above nine 

 and science required those of larger size, the introduction of mochiin'ry 

 was necessary to progress. The two Herschels availed themselves of 

 mechanical aid; but the first astronomer who has given a minute 

 account of his method ia Lord Rosse, in the paper above referred to. 

 His plan is to work by a double system of cross-strokes, while the 

 speculum and polisher are slowly revolving with unequal velocities, 

 The speculum is carried by a chuck attached to a strong vertical shaft, 

 and is surrounded by water at 55 to insure the proper consistence of 

 the pitch. The polisher is of iron, stiffened by means of ribs arranged 

 like the walls of a honeycomb, and suspended by six point* to lessen 

 the chance of bending. The poUshing-bar has a rectilinear motion 



from a variable crank. The effect of thi* crank corresponds to the 

 cross stroke in hand-polishing, tending to a spherical form ; but tha 



