6 ON THE CONSTRUCTION AND USE OF 



can be accomplished as follows : Take three grains of hyposulphite of soda, and 

 dissolve it in an ounce of water. Add to it slowly a solutif)n in water of one grain 

 of chloride of gold. A lemon yellow liquid results, which eventually becomes clear. 

 Immerse the silvered glass in it for twentj-four hours. An exchange will take place, 

 and the filui l^ocome yellowisli. I have a piece of glass prepared in this way which 

 remains unhurt in a box, where other pieces of plain silvered glass have changed 

 some to yellow, some to blue, from exposure to coal gas. 



I have also used sihered glass plates for daguerreotyping. They iodize beauti- 

 fully if freshlv })olifihed, and owing jirobably to the absence of tlie usual copi)('r 

 alloy of silver plating, take impressions Avitli very short exposures. The resulting 

 picture has a rosy warmth, rarely seen in ordinary daguerreotypes. The only pre- 

 caution necessary is in fixing to use an alcoholic solution of cyanide of potassiiun, 

 instead of liyposulphite of soda dissolved in wat(>r. The latter has a tendency to 

 split up the siher. The subsequent Avashing must be with dilutcMl comuiou alcohol. 



Pictures obtained by tliis method will bear high nmgnifying powers without 

 showing granidation. Unfortunately the exposure required for tliem in the telescope 

 is six times as great as for a sensitive wet collodion, though the iodizing be carried 

 to a lemon )cllow, the bromizing to a ros(> red, and the ])late be returnetl to the 

 iodine. 



(;}.) (i^RIN-niNO AND POLTSHING Gl.\SS. 



SouK^ of tlie facts stated in the following paragra])lis, the result of numerous 

 experiments, may not be new to practical opticians. 1 Iia\c liad, however, to polish 

 with my oavu hands more tluin a liundred mirrors of various sizes, from 19 inches 

 to \ of an inch in diameter, and to experienc(> very frecpient failun^s for three years, 

 before succeeding in producing large surfaces with ctn'tainty and (piickly. It is 

 well nigh impossible to obtain from (ii)ti(iaiis the practical minutia^ whicli arc 

 essential, and which they conceal even from each other. 'I'lie long continued re- 

 searches of Jjord Rosse, Mr. Lassell, and M. I'oucauU ;ire full of the most \alualile 

 facts, and have been of continual use. 



The subject is divided into: a. The Peculiarities of (ilass ; b. Emery and Rouge; 

 0. Tools of Iron, Lead and Pitch ; d. Methods of Examining Surfaces; e. Machines. 



a. I'l i-iiliii riliix <if (iJafis. 



Effects of Pressure. — It is generally supposed that glass is possessed of tlie power 

 of resistance to compression and rigidity in a very marked manner. In the course 

 of tlies(> experiments it has appeared that a sheet of it, even when very thick, can 

 with difficulty be set on edge without bending so much as to be optically worthless. 

 Fortunatclv in every disk of "lass that I have tried, there is one diameter on eitlnn- 

 end of which it may stand without harm. 



In examining lately \arious Avorks on astronomy and optics, it appears that the 

 same difficulty has been found not only in glass but also hi speculum mental. Short 

 used always to mark on the edge of the large mirrors of his Gregorian telescopes 

 the point AA^hich should be placed uppermost, in case they A\-ere removed from their 

 cells. In achromatics the image is Aery sensibly changed in sharpness if the flint 



