CHAP. I.] RESILVERING MIRRORS 11 



The telescope should be of a high magnifying power and Good Tube 

 1 J n -J- Invalu- 



clear dennition. able. 



These sextants are now supplied by the Hydrographic Office 

 with two telescopes — one for ordinary use, and another, of 

 aluminium, with a larger object glass for occasions when faint 

 objects are required to be seen. The collimation of these large 

 telescopes is however a delicate matter, and when accuracy is 

 required, should be tested. 



When in good adjustment, a sounding sextant so fitted is 

 invaluable for star observations with a faint sea horizon. 



RESILVERING MIRRORS. 



On service, the mirrors of sextants, especially sounding sex- 

 tants, frequently get dimmed by damp, and the surveyor must 

 be able to resilver them himself. 



A supply of tinfoil, of good quality, for this purpose, is one 

 of the necessary stores. Mercury is always to be had. The 

 operation has been frequently described, but it is perhaps better 

 to repeat it. 



Take a piece of tinfoil, a httle larger than the glass to be 

 silvered, and smooth it out on a perfectly flat surface, as a 

 sheet of plate glass, or a thick smooth book-cover. This 

 smoothing can be weU done by a Httle pad of chamois leather, 

 which can be kept for the purpose, or by the finger. 



Drop a small bubble of mercury on to the foil, and by gentle 

 rubbing with the pad, spread it over the former so that it shows 

 a bright surface. Pour mercury on until the piece of foil is 

 quite fluid, and brush any large spots of dross lightly off. Lay 

 a piece of clean paper, long enough to handle easily, on the 

 mercury, and the glass, previously well cleaned by means of 

 spirits of wine, on the paper. Pressing on the glass with one 

 hand, withdraw the paper with the other, slowly and steadily, 

 and a pure surface will appear under the glass, the dross all 

 coming away with the paper. 



Incline the book, or whatever surface we have been working 

 on, so as to let superfluous mercury run off, placing strips of 

 tinfoil at the lower edge to assist in sopping this up. 



After from twelve to twenty-four hours, the amalgam will 

 be dry, and firmly adhering to the glass. Out the edges care- 



