116 SIR JOHN FREDERICK WILLIAM HERSCHEL. 



They had each a clear diameter of I84 iuches of polished surface, and 

 \\ ere all equally reflective wheu freshly polished, and perfectly similar 

 in their perfori^iance. The operation of re-polishing, which was more 

 frequently required than in England, was performed by himself with 

 the requisite apparatus, Avhich he also brought from England. 



Although Sir John Herschel never exhibited — as indeed he had no 

 occasion to do — the wonderful mechanical genius of his father, he never- 

 theless fully understood all the former's methods of preparing and treating 

 specula. When it was stated at a meeting of the British Association in 

 1842, that Lord Eoss had attained such skill in the treatment of metallic 

 specula that he could dismount the mirror of his large telescope, 

 repolish it, and replace it the same day. Sir John four years previously 

 had written to Arago these words: ''By following my father's rules 

 miuutely and using his apparatus, I have succeeded in a single day, 

 without the least assistance, in polishing completelj" three Newtonian 

 mirrors of nineteeu-iuch aperture," 



In the use of reflecting specula of considerable weight, it is of the 

 utmost importance that the metal should be supported in its case so as 

 not to sufler uny change of figure from its own weight. Sir John found 

 that a speculum was totally useless by allowing it to rest horizontally 

 on three metallic points at its circumference. The image of every con- 

 siderable star became triangular^ throwing out long flaming caustics at 

 the angles. Having on one occasion supported the speculum simply 

 against a flat board, inclined at an angle of about 45°, he found that its per- 

 formance was tolerably good ; but on stretching a thin pack-thread verti- 

 cally down the middle of the board, so as to bring the weight of the metal 

 to rest upon the thread, the images of the stars were lengthened hori- 

 zontally " to a preposterous extent, and all distinct vision utterly de- 

 stroyed by the division of the mirror into two lobes, each retaining 

 something of its i)arabolic figure, separated by a vertical band in a state 

 of distortion, and of no figure at all ! " The method which Sir John 

 found the best was the following : Between the mirror and the back of 

 the case he interj)osed six or seven folds of thick woolen baize, of 

 uniform thickness and texture, stitched together at their edges. The 

 metal, when laid flat on this bed, Avas shaken so as to be concentric 

 with the rim of the case, and two sui)ports, composed of several strips 

 of similar baize, were introduced so as to occupy about 30"^ each, and 

 to leave an arc of about 40'^ unoccupied opposite the point which was 

 to be lowermost in the tube. "When the case is raised into an inclined 

 position, and slightly shaken, the mirror takes its own free bearing on 

 these supports, and preserves its figure. It is essential, however, to 

 the successful application of this method that many thicknesses of the 

 . baize should be employed, by which only the etiect of flexure in the 

 wooden back of the case can be eliminated." 



This simple plan, adopted by Sir John Herschel, is mentioned to 

 show how mechanical genius aided him, as it did his father before 



