an autobiography-第83节
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ll cases proved most handy and satisfactory。
This method consisted of tinning three； or； if need be； more parts of the work； and laying them down on a tinned face…plate or chuck； which had been heated so as just to cause the solder to flow。 As soon as the solder is cooled and set； the chuck with its attached work may then be put in the lathe； and the work proceeded with until it is completed。 By again heating the chuck； by laying upon it a piece of red…hot iron； the work； however delicate； can be simply lifted off； and will be found perfectly free from all distortion。
I have been the more particular in naming the use of three points of attachment to the chuck or face…plate； as that number is naturally free from any risk of distortion。 I have on so many occasions found the great value of this simple yet most secure mode of fixing delicate work in the lathe； that I feel sure that any one able to appreciate its practical value will be highly pleased with the results of its employment。
The same means can； in many cases； be employed in fixing delicate work in the planing…machine。 All that is requisite is to have a clean…planed wrought…iron or brass fixing…plate； to which the work in hand can be attached at a few suitable parts with soft solder； as in the case of the turning lathe above described。
1828。 A Method of casting Specula for Reflecting Telescopes； so as to ensure perfect Freeness from Defects； at the same time enhancing the Brilliancy of the Alloy。
My father possessed a very excellent achromatic spy…glass of 2 inches diameter。 The object…glass was made by the celebrated Ramsden。 When I was about fifteen I used it to gaze at the moon； planets； and sun…spots。 Although this instrument revealed to me the general characteristic details of these grand objects； my father gave me a wonderful account of what he had seen of the moon's surface by means of a powerful reflecting telescope of 12 inches diameter； made by Short that justly celebrated pioneer of telescope making。 It had been erected in a temporary observatory on the Calton Hill； Edinburgh。 These descriptions of my father's so fired me with the desire to obtain a sight of the glorious objects in the heavens through a more powerful instrument than the spy…glass； that I determined to try and make a reflecting telescope which I hoped might in some degree satisfy my ardent desires。
I accordingly searched for the requisite practical instruction in the pages of the Encyclopedia Britannica； and in other books that professed to give the necessary technical information on the subject。 I found； however； that the information given in booksat least in the books to which I had access was meagre and unsatisfactory。 Nevertheless I set to work with all earnestness； and began by compounding the requisite alloy for casting a speculum of 8 inches diameter。 This alloy consisted of 32 parts of copper； 15 parts of grain tin； and 1 part of white arsenic。 These ingredients； when melted together； yielded a compound metal which possessed a high degree of brilliancy。 Having made a wooden pattern for my intended 8…inch diameter speculum； and moulded it in sand； I cast this my first reflecting telescope speculum according to the best book instructions。 I allowed my casting to cool in the mould in the slowest possible manner； for such is the excessive brittleness of this alloy （though composed of two of the toughest of metals） that in any sudden change of temperature； or want of due delicacy in handling it； it is very apt to give way； and a fracture more or less serious is sure to result。 Even glass； brittle though it be； is strong in comparison with speculum metal of the above proportions； though； as I have said； it yields the most brilliant composition。
Notwithstanding the observance of all due care in respect of the annealing of the casting by slow cooling； and the utmost care and delicate handling of it in the process of grinding the surface into the requisite curve and smoothness suitable to receive the final polish； I was on more than one occasion inexpressibly mortified by the sudden disruption and breaking up of my speculum。 Thus many hours of anxious care and labour proved of no avail。 I had to begin again and proceed da capo。 I observed； however； that the surplus alloy that was left in the crucible； after I had cast my speculum； when again melted and poured out into a metal ingot mould； yielded a cake that； brittle though it might be； was yet strong in comparison with that of the speculum cast in the sand mould； and that it was also； judging from the fragments chipped from it； possessed of even a higher degree of brilliancy。
The happy thought occurred to me of substituting an open metal mould for the closed sand one。 I soon had the metal mould ready for casting。 It consisted of a base plate of cast iron； on the surface of which I placed a ring or hoop of iron turned to fully the diameter of the intended speculum； so as to anticipate the contraction of the alloy。 The result of the very first trial of this simple metal mould was most satisfactory。 It yielded me a very perfect casting： and it passed successively through the ordeal of the first rough grinding； and eventually through the processes of polishing； until in the end it exhibited a brilliancy that far exceeded that of the sand mould castings。
The only remaining difficulty that I had to surmount was the risk of defects in the surface of the speculum。 These sometimes result from the first splash of the melted metal as it is poured into the ring mould。 The globules sometimes got oxidised before they became incorporated with the main body of the inflowing molten alloy： and dingy spots in the otherwise brilliant alloy were thus produced。 I soon mastered this； the only remaining source of defect； by a very simple arrangement。 In place of pouring the melted alloy direct into the ring mould； I attached to the side of it what I termed a 〃pouring pocket；〃 which communicated with an opening at the lower edge of the ring； and by a self…acting arrangement by which the mould plate was slightly tilted up； the influx of the molten alloy advanced in one unbroken tide。 As soon as the entire surface of the mould plate was covered by the alloy； its weight overcame that of my up…tilting counterpoise； and allowed the entire apparatus to resume its exact level。 The resulting speculum was； by these simple arrangements； absolutely perfect in soundness。 It was a perfect casting； in all respects worthy of the care and labour which I invested in its future grinding and polishing； and enabled it to perform its glorious duties as the grand essential part of a noble reflecting telescope！
＇Image＇
A。 Chill plate of cast iron turned to the curve of the speculum B。 Turned hoop of wrought iron with opening at O。 C。 Pouring pocket。 D。 Counterpoise； By which the chill plate is tilted up The largest figure in the engraving is the annealing tub of cast iron filled with sawdust； where the speculum is placed to cool as slowly as possible。
The rationale of the strength of specula cast in this metal mould system； as compared with the treacherous brittleness of those cast in sand moulds； arises simply from the consolidation of the molten metal pool taking place first at the lower surface； next the metal base of the mouldthe yet fluid alloy above satisfying the contractile requirements of that immediately beneath it； and so on in succession； until the last to consolidate is the top or upper stratum。 Thus all risk of contractile tension； which is so dangerously eminent and inherent in the case of sand…mould castings； made of so exceedingly brittle an alloy as that of speculum metal； is entirely avoided。 By the employment of these simple and effective improvements in the art of casting the specula for reflecting telescopes； and also by the contrivance and employment of mechanical means for grinding and polishing them； I at length completed my first 8…inch diameter speculum； and mounted it according to the Newtonian plan。 I was most amply rewarded for all the anxious labour I had gone through in preparing it； by the glorious views it yielded me of the wonderful objects in the heavens at night。 My enjoyment was in no small degree enhanced by the pleasure it gave to my father； and to many intimate friends。 Amongst these was Sir David Brewster； who took a most lively and special interest in all my labours on this subject。
In later years I resumed my telescope making enjoyments； as a delightful and congenial relaxation from the ordinary run of my business occupations。 I constructed several reflecting telescopes； of sizes from 10…inch to 20…inch diameter specula。 I had also the pleasure of assisting other astronomical friends； by casting and grinding specula for them。 Among these I may mention my late dear friend William Lassell； and my excellent friend Warren de la Rue； both of whom have indelibly recorded their names in the annals of astronomical science。 I know of no subject connected with the pursuit of science which so abounds with exciting and delightful interest as that of constructing reflecting telescopes。 It brings into play every principle of constructive art； with the inexpressibly glorious reward of a mor
