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 solution 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 twenty-four hours. An exchange will take place, 

 and the film become yellowish. 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 silvered glass plates for daguerreotyping. They iodi/e beauti- 

 fully if freshly polished, and owing probably to the absence of the usual copper 

 alloy of silver plating, take impressions with 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 potassium, 

 instead of hyposulphite of soda dissolved in water. The latter has a tendency to 

 split up the silver. The subsequent washing must be with diluted common alcohol. 



Pictures obtained by this method will bear high magnifying powers without 

 showing granulation. Unfortunately the exposure required for them in the telescope 

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

 to a lemon yellow, the bromizing to a rose red, and the plate be returned to the 

 iodine. 



(3.) GRINDING AND POLISHING GLASS. 



Some of the facts stated in the following paragraphs, the result of numerous 

 experiments, may not be new to practical opticians. I have had, however, to polish 

 with my own hands more than a hundred mirrors of various sizes, from 19 inches 

 to % of an inch in diameter, and to experience very frequent failures for three years, 

 before succeeding in producing large surfaces with certainty and quickly. It is 

 well nigh impossible to obtain from opticians the practical miuutitr which are 

 essential, and which they conceal even from each other. The long continued re- 

 searches of Lord Rosse, Mr. Lassell, and M. Foucault are full of the most valuable 

 facts, and have been of continual use. 



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

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



a. Peculiarities of Glass. 



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

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

 of these 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. 

 Fortunately in every disk of glass that I have tried, there is one diameter on either 

 end of which it may stand without harm. 



In examining lately various works on astronomy and optics, it appears that the 

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

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

 the point which should be placed uppermost, in case they were removed from their 

 cells. In achromatic* the: image is very sensibly changed in sharpness if the flint 



