"How much do you think we ought to make up at first?"
"Several gallons of the sand will do for the experiment."
"What kind of article should be made with the first trial?"
"We might make some window glass. It is true it will not be transparent, but it will be translucent, and so will give us light, as well as though it should be transparent."
"What is translucent glass?"
"Where the surface of a cast plate is polished the material is such that you can see through it, but if it is left rough it is impossible to see through it, although it will permit light to go through. The term applied to such glass is translucent."
"If light will pass through, why is it the eye cannot see through it?"
"A powerful magnifying glass shows that the surface of unpolished glass is formed by a layer of crystals, or of sand, with the faces projecting out in all directions and at all angles. The result is, that a beam of light from the eye strikes one or more of these faces and is diverted from a straight line through the glass. As all the rays are thus changed from a direct course, confusion results, and the eye distinguishes nothing."
Several bushels of the sand were brought to the laboratory, and the Professor then directed the preparation of a half dozen slate slabs, each slab being nearly two feet square. He explained that in practice iron plates were used, but as they had nothing of that kind available, slate would answer admirably.
"The slate slabs must be heated, and when the fused material is poured on the slabs, the heat must be kept up for a short time and gradually cooled down."