MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
7 
powerful but very small double convex lens of a rare substance in a very dense 
medium, or as equally deep double concave lenses of glass would do in air ; 
they rapidly, therefore, turn the rays impingeing on them on one side, and 
occasion a loss of light, just as so many opaque spots would do. But as even 
when numerous their united area may amount to only a very small propor- 
tion of the area of the plate of glass required for a telescope, this loss of light 
is usually of but little consequence. In practice, it is said that no other real 
evil than such loss of light is dependent on them. 
5. Of all these faults, that of the irregularity constituting streaks, strise, and 
waves, is the most difficult to avoid, and the most injurious in its effect. It is 
not an improvement only beyond 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 atten- 
tion has been paid. The reason of this will be evident, if the general compo- 
sition of the two glasses be taken into account. The required difference be- 
tween 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 difference 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 heat- 
ed state for many hours, it does not dissolve much from it; and what it does 
dissolve having a refractive power little different from that of the glass itself, 
proportionately less harm is occasioned. Again: the specific gravity of the dif- 
ferent materials used is not veiy 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 energetical- 
ly exerted, and the whole mass approaches nearer to uniformity in a given time, 
or acquires it sooner than would happen were greater differences to exist. 
