[94] An achromatic lens is one that is free from color.
Flint, or lead, glass melts more easily than crown, or common, glass and this is a good pointer for you to remember when you are getting glass for your glass blowing experiments. Bohemian glass, which is largely used for chemical apparatus, is made of sand, lime and potash.
Colored glass is made by putting small quantities of various substances into the melted glass. Thus oxide of cobalt[95] gives a blue color; oxide of chromium, or cupric oxide a green; one of the copper oxides gives it a red color, uranium[96] a yellow, etc.
[95] Cobalt is a metal and is usually found along with nickel.
[96] Uranium is also a metal.
How to Cut Glass.
—About the simplest and most useful process for you to know is how to cut glass.
To cut window glass you need (1) a glass cutter; (2) a drawing board and (3) a T square. There are two kinds of glass cutters on the market and these are (a) steel cutters and (b) diamond cutters.
The first kind has a little steel wheel in the end, as shown at [A in Fig. 83]. You can buy one for a quarter and it will serve all your needs very well. If money is no object you can buy a glass cutter with a diamond point such as glaziers use for about $4.00. This kind of a glass cutter has a chip of genuine diamond in a swivel end and it is a pleasure to cut glass with one of them. It is shown at [B].
