238 On the Manufacture of Optical Glass. [1829. 



most injurious in its effect. It is not an improvement only be- 

 yond what is ordinarily done in this respect that is required, 

 but absolute perfection, a homogeneity equal to that of pure 

 water. In the two kinds of glass required to render a telescope 

 achromatic, namely, crown or plate glass, and flint glass, it is 

 the latter which is obtained perfect with the greatest difficulty, 

 and to which therefore the greatest attention has been paid. 

 The reason of this will be evident, if the general composition 

 of the two glasses be taken into account. The required differ- 

 ence between them in refractive and dispersive power is found 

 to be at command, by attention to composition ; and it has been 

 also ascertained, that crown and plate glass answer exceedingly 

 well for the one variety, and flint glass for the other. Crown 

 glass consists of silica, lime, oxide of iron, sometimes a little 

 alkali, and small quantities of other matters : these substances 

 are not very different in their refractive powers, and when fused 

 do not produce very strong streaks, even though a little differ- 

 ence in the composition of different parts of the glass may exist. 

 The glass also is not a very powerful fluxing agent upon the 

 crucible in which it is melted ; so that although it is in contact 

 with it in a fluid and heated state for many hours, it does not 

 dissolve much from it ; and what it does dissolve having a re- 

 fractive power little different from that of the glass itself, propor- 

 tionately less harm is occasioned. Again : the specific gravity 

 of the different materials used is not very different ; so that the 

 mixing agencies which affect the contents of the pot, such as 

 the ascent of bubbles, the ascending and descending currents 

 from difference of temperature, are more energetically ex- 

 erted, and the whole mass approaches nearer to uniformity in 

 a given time, or acquires it sooner than would happen were 

 greater differences to exist. 



6. With plate glass the same circumstances hold nearly in an 

 equal degree. This substance is composed of silica and alkali 

 essentially, other elements being only in small quantities. Its 

 action upon the crucible is greater than crown glass, but then 

 it has a second application of heat in such circumstances as are 

 calculated to give a very uniform temperature to the contents 

 of a whole pot, and it is delivered into its final form in the 

 manner least likely to cause mixture of the different parts. 



7. With flint glass many circumstances are altogether differ- 



