A SILVERED GLASS T I-; T. E SCOP E . 



IH 



It is well, on commencing to polish with a tool niiiilc in this way, to warm the 

 glass as well as the tool in water (page -i) before bringing the two in contact. If 

 this is not done the polishing will not go on kindly, a good adaptation not being 

 secured for a length of time, and the glass surface being injured at the outset. The 

 rosin on a polislier put away for a day or two suffers an internal cliangc, a species 

 of irregular swelling, and does not retain its original form. Heating, too, has a 

 good effect in preventing disturbance by local variations of temperature in the glass. 



The description of " Local Polishers" will be gixcn under Machines. 



d. Methods of Examhiiiuj Surfaees. 



I have been in the habit of testing mirrors (exclusively at the centre of curvature, 

 not putting them in th(> telescope tube until nearly parabolic or finished. 'I'hc 

 means of trial are so excelhnit, the indications obtained so precise, and the freedom 

 from atmospheric disturl)ances so complete, that tlie greatest facilities are offered 

 for ascertaiinng the nature of a surfcice. In addition the observer is entirely inde- 

 pendent of day or night, and of the weather. I do not think that anything more 

 is learned of the telesco])e, even under favorable circiunstances, than in the work- 

 shop. For tlie improxciiient of these methods of ol)ser\ati()n, Science is largely 

 indebted to M. Foucault, wliose tliird test — the- second in the next paragraph— is 

 sufficient to afford by itself a large part of the informatidii retpiired in correcting 

 a concave surface. 



There are two distinct modes of examination: 1st, observing with an eye-piece 

 the image of an illuminated piu-lioU' at the focus, and tlie coni> of rays inside and 

 outside that plan(> ; 2d, recei\ing tlie entire pencil of light c(uning from the mirror 

 through the pupil on the retina, and noticing the distribution of light and shade, 

 and the appearances in relief on the face of the mirror. 



The arrangements for these tests are as follows : Aromid the flame of a lamp {a. 



Fig. 8. 



Testing a Concave at the Centre of Curv.Tture. 



Fig. 8) a sheet of tin is bent so as to form a cylindrical schmmi. Tlirough it ;it tlie 

 height of the brightest part of the flame, as at h, two holes are bored, a (|uarter of an 

 inch apart, one J^ of an inch in diameter, the other as small as the point of tbe tiiiest 

 needle will make— perhaps ^^o o^" ''^^ i"*-'^'- '^'""^ apparatus is to be set at the centre 



