the Colouring of Glass hy Metallic Oxides. 4-4. 5 



The various facts observed in the colouring of glass, which 

 are especially produced by the influence of different tempera- 

 tures, are probably to be attributed to some modifications in 

 the disposition of the composing particles', which effects occasion 

 modifications in the reflexion and refraction of the rays of 

 light : indeed it might be remarked, that parts of the results 

 which I have meniionec] are produced tmder some circumstances 

 which appear to place the glass in a condition of crystallization. 



In the last century, Edward Hussy Delaval, starting from 

 the experiments made by the immortal Newton in the colouring 

 of thin plates, instituted some researches into the causes of the 

 modifications of colours in bodies; but he found chemical 

 science not in a state sufficiently advanced to establish his 

 observations on rational experiments. But at the present 

 lime we have only to collect a sufficient number of precise 

 facts to be able to deduce from them the scientific explana- 

 tions, which might probably lead to some new improvements 

 in manufactures. 



As for glass, the observations relating to the constitution of 

 its particles are extremely delicate. This is proved by the 

 difference of the action of light on it, according to the degree of 

 annealing. It is known that even a very slight pressure, acting 

 on a point of its surface, is sufficient to produce the doubly re- 

 fracting power, which is also given by incomplete annealing; 

 and this effect takes place, not only when the glass, having 

 been quickly cooled from a red heat to the ordinary tempera- 

 ture, is liable to break by itself, but even in pieces of glass of 

 some thickness, which might be considered to be well annealed, 

 and which would really be sufficiently annealed for common 

 use : it is a fact, that the greatest part of such a glass shows 

 sensibly the phaenomena of polarization. This fact has still 

 increased the difficulties, which were already very great, in 

 manufacturing glass for optical purposes. The difficulty, 

 which is not a small one for discs of three or four inches in 

 diameter, is of course greater for discs often and twelve inches ; 

 we have however surmounted it at Messrs. Chance's glass 

 works for discs up to twenty-four inches : but before working 

 such discs, or larger ones, we think that it would be desirable 

 that practical opticians should throw sufficient light on the 

 various parts of the processes which are used in the construc- 

 tion of achromatic telescopes; because we could not warrant 

 that the glass which we consider to be free from defects, may 

 not, with very high magnifying powers, give evidence of new 

 imperfections which we have not yet suspected. 



I have laid before you practical facts. If they be found 

 interesting enough to form the basis of new studies on the 



