EYE-PRESERVING GLASS FOR SPECTACLES. 
5 
apparatus for cutting, grinding, and polishing was also necessary so that the 
synthetically made glasses could be cut into plates—polished so as to be tested photo¬ 
graphically in the spectrograph already described—and also tested for the percentage 
of heat rays they obstructed. 
Many preliminary experiments were made on the preparation of a clear and colour¬ 
less glass or flux to serve as a basis for the colouring with the various metallic oxides- 
Finally, two kinds of soda glass not containing lead were chosen, and Mr. H. Powelb 
of the Whitefriars Glass Works, who had assisted me in the preliminary trials by 
supplying me with many kinds of glass of different composition and fusibility, made a 
quantity of these fluxes and supplied them in a crushed condition. In my earlier 
laboratory experiments the mixture of colouring matter and granulated flux was put 
into a small “ gold pot of Morgan’s Crucible Co., and gradually heated over a 
“ Meker ” gas burner. It is advisable to have one at least of the colouring constituents 
in the form of nitrate so that its decomposition by heat shall mix and stir the 
constituents. The decomposition of the nitrate causes a little frothing ; therefore it is 
necessary to add the mixture gradually to the crucible, to avoid frothing over. When 
all is added and the contents well fritted, the hot crucible is removed to an electic 
furnace and the temperature slowly raised until the glass is quite fluid. It is stirred 
at frequent intervals with a stout platinum rod. After an hour the stirring is 
discontinued, and the temperature kept up for an hour and a-half. The current is then 
cut off, the openings in the furnace plugged with asbestos to prevent draughts, and the 
whole allowed slowly to cool to anneal the glass. In some cases the composition of 
the glass was such that the melting-point had to be raised above 1400° C., and as this 
temperature was beyond the safe limit with the platinum strip furnace, a blast-furnace 
fitted with a “ Lennox ” electric blower was used; with this arrangement larger 
quantities of glass could be raised with safety to a much higher temperature. 
There are two conditions I have endeavoured to secure of the finished glass—each 
of great importance. One, the most essential, is the absence of all streaks, strise, and 
irregularities of density ; the other, the absence of air-bubbles. The first is obtained 
by repeated stirring and perfect admixture ; the freedom from air-bubbles is secured b}^ 
leaving the glass in perfect repose while the heat is at the highest point. On these 
and other points 1 have been much aided by reading an early paper by Faraday, “ On 
the Manufacture of Glass for Optical Purposes,” the Bakerian Lecture read before the 
Royal Society in 1829 (‘ Phil. Trans. Rny. Soc.,’ 1830, p. l). On a small scale it is 
almost impossible to avoid slight striae owing to differences of density caused by the 
long continued heat volatilising some of the soda. Faraday was much harassed by 
this dilemma in the manufacture of his optical glass, and tried many experiments to 
ascertain the cause. To get rid of air-bubbles Faraday used spongy platinum in 
powder sprinkled over or added to the bulk of the melted glass. This was found to 
act pretty well, making the bubbles rise in the same manner as a piece of bread causes 
bubbles to rise when thrown into a glass of effervescing liquid. To get the full benefit 
