34 Dr Goring on Reflecting Microscopes. 



not that if its figure was perfect while he was working it at- 

 tached to the handle, it would undergo a change when detached 

 from it* and he would be compelled to make it of considerable 

 thickness at last, to insure the stability of its figure, and its ad- 

 justment when fixed in the tube. 



Reflecting instruments are proportionally much worse in their 

 performance than refractors, if in the least out of adjustment ; 

 and the adjustment of very small metals to each other is an ope- 

 ration of almost inconceivable delicacy. For this purpose, I 

 repeat, as well as to form a handle, the piece of brass is soldered 

 to their back, and the metal and its support, whicli has a screw 

 behind to fix it to the chuck, turned very true together, at the 

 same chucking in the lathe in which the figure of the metal 

 also is roughly formed, so that the centering of the metal may 



• The late Mr William Tulley told me the following anecdotes of me- 

 tals : — He had polished a Gregorian metal of five inches aperture in the 

 evening, cleaned it, and placed it in its tube. He then allowed it to stand 

 in the open air till he thought it had acquired the temperature of the atmo- 

 sphere, and on trying it, was perfectly delighted with its performance. He 

 left it out, and the next morning he got up before daylight to try it again, 

 and found it one of the poorest things imaginable ! By that time it had indeed 

 come to the temperature of the air, and consequently changed its figure 

 which it had not, it seems, in the night before. Sir J. Herschell once told 

 me, if I remember rightly, that his large telescopes required fully four hours 

 exposure to come to the temperature of the air ; they then perform as well 

 s small ones. 



Mr Tulley likewise related to me that he was once engaged a whole week 

 in figuring a metal of three inches aperture and six focus for a dumpy Gre- 

 gorian ; when he had succeeded after infinite trouble, some one burst sud- 

 denly into the room where he had been employed on it, violentli/ kicking the 

 door open. The metal, which was of the ))est composition and well annealed, 

 instantly broke into two pieces, apparently from the concussion produced 

 in the atmosphere of the room, just as windows and mirrors are cracked by 

 the fire of artillery. 



Mr Charles Tulley observed to me that one day he was viewing the nails 

 on the dome of St Paul's with a Newtonian telescope placed in the sun, and 

 the diagonal metal of which was about one-eighth of an inch thick, sol- 

 dered to a piece of tube, without any suppoH at the back of the speculum ; he 

 found, though the fi<-ure of the plane was good, that it shewed the said 

 nails oval (their heads are round), and distorted the figures of other things 

 proportionally. The rarefaction of the air in the tube of tlie telescope had 

 been sufficient to alter the figure of the jjlane mirror which the rays of the 

 sun could not get at ; had it been made of a solid lump of metal, the effect, 

 Mr Tulley said, would not have beea produced. 



