70 ANNUAL OF SCIENTIFIC 



spots and foci in their variations of intensity, and the less luminous 

 marginal regions, are shown with the <rreate.-t clearness, and 

 oven the filmiest clouds and vapors which >eem to sweep over the 

 disk can he readily perceived. The examination can lie kept up 

 any length of time without strain to the e\, 3, The physiological 

 influence is very different from that of colored glasses, the use of 

 which is sometimes very objectionable. Since all the different 

 rays of light pass through the metal (although greatly tempered) 

 except the outermost red rays, which are excluded, together with 

 the dark heat rays, the silver must he deposited in the usual gal- 

 vanoplastic or chemical manner, so as to form a verv delicate 

 film. Gold and platinum may also be used, but silver possesses 

 several advantages. 



This property, on the part of metals, of greatly subduing the rays 

 of light, without extinguishing them to any extent, and of exclud- 

 ing almost entirely the rays of heat, is now applied to other prac- 

 tical purposes. Weak eyes can use spectacles thus prepared to 

 the greatest advantage, where colored glasses are not to be 

 thought of. For persons who.e business keeps them before :t 

 glowing lire, such glasses are invaluable, since the .-iu'ht is not 

 strained by the light, nor the eyeball injured by'the heat, which 

 is measurably excluded. Screens of glass, to be placed before 

 iires, have also been made on the same principle. 



MIKKORS WITHOUT MEUlTllV. 







It is well known that of the manufacturing arts, that of prepar- 

 ing glass for mirrors is one of the most injurious to the health of 

 the artisan. Hitherto science has failed to surest any method 

 of defending those engaged in this dangerous employment i'roiu 

 the poisonous exhalations of mercury, which is used in large 

 quantities in this manufacture; but now French ingenuity has 

 given to the world a substitute, which bids fair to supersede t ho- 

 use of mercnrv entirely, and make of a dreaded and fatal art one 



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wholly innocent, as well as agreeable and cleanly. The report 

 of M. Salvetat to the Society of Knc-ouragement in Paris describes 

 and approves this invention, for which it predicts a great success. 



This invention is what may be called a method for the metalli- 

 zation of glass of every kind, even the coarsest and most ordinary, 

 which, by a rapid, simple, and inexpensive process, becomes an 

 excellent reflecting medium, while, strange to say, it still retains 

 its transparency, so that the same glass may answer at the same 

 time tin 1 double purpose of window and mirror. 



The ordinary method of preparing looking-glasses is with an 

 amalgam of tin anil mercury, four parts of tin to one of mercury. 



In the invention reported by M. Salvetat, neither mercury nor 

 tin is used at all. The tinfojl is replaced by platina, not applied 

 in leaf form, of course, but chemically, in a metallic and brilliant 

 powder. The operation is perfectly Dimple. The glass, having 

 been carefully cleaned and polished, is covered, by means of a 

 brush, with a mixture of chloride of platina, essence of lavender, 

 and a dissolvent composed of litharge and borate of lead. When 



