252 Sir J. F. W. Herschers Address to the 



have had experience of the annoyance of having to keep an 

 object long in view, especially under high magnifying powers, 

 and in micrometrical measurements, with a reflector mounted 

 in the usual manner, having merely an altitude and azimuth 

 motion, can duly feel and appreciate the advantage thus 

 gained. But the difficulties to be surmounted in the execu- 

 tion of such a mode of mounting were very considerable — 

 much more so than in the case of an achromatic, — owing 

 partly to the non-coincidence of the centre of gravity of the 

 telescope and mirror with the middle of the length of the 

 tube, and partly to the necessity of supporting the mirror it- 

 self within the tube in a uniform bearing free from lateral 

 constraint, and guaranteed against flexure and disturbance of 

 its adjustment by alteration of its bearings. These difficul- 

 ties, however, Mr Lassell overcame : the latter which is the 

 most formidable, by an ingenious adaptation of the balancing 

 principle, first devised, if I am not mistaken, by Fraunhofer 

 and Reichenbach for the prevention of flexure in the tubes of 

 telescopes — a principle which has not received half the ap- 

 ydications of which it is susceptible, and which, by throwing 

 the whole strain of the weight of instruments on axes which 

 may be made of unlimited strength, may be employed to de- 

 stroy the distorting force of gravity on every other part.* 



The success of this experiment was such, and the instru- 

 ment was found to work so well, that Mr Lassell conceived 

 the bold idea of constructing a reflector of two feet in aper- 

 ture and twenty feet in focal length, and mounting it upon 

 the same principle. The circumstances of his local situation, 

 in the centre of manufacturing industry and mechanical con- 

 struction, were eminently favourable to the success of this 

 undertaking ; and in Mr Nasmyth he was fortunate enough 

 to find a mechanist capable of executing in the highest perfec- 

 tion all his conceptions, and prepared, by his own love of as- 

 tronomy, and practical acquaintance with astronomical obser- 



* As, for example, the divided limbs of circles, and the spokes connecting 

 them with their centres; an easy and simple mechanism, which, devised some 

 time ago, and approved by the late M. Bessel, I may, perhaps, take some fu- 

 ture opportunity to submit to the Society.— (iVbfe added in the Frinting.) 



