But if both sides are rounded out, or if one side is rounded out and the other side is flat, it turns all the rays a little, and brings them right together in a point not far from the glass.

If the sides are hollowed _in_ instead of bulging out, the rays scatter, and if one side bulges out and the other bulges in, as they do in a watch crystal, one side scatters and the other side collects the rays, and so it is the same as if the glass had been perfectly flat, one side undoes the other's work.

Now I have no glass which bulges out on both sides, and none that bulges out on one side and is flat on the other, but my watch crystal bulges out on one side and in on the other.
But when I filled it with water, the water being as clear as the glass, it made it flat on top and bulging underneath, and so it gathered the sun's rays together in the light spot you saw, and set fire to the cotton." "Yes, but why did you have to wait till noon ?" asked Tom.
"Because the glass must be held right across the rays of light, and as I couldn't turn the crystal to either side without spilling the water, I had to use it at noon, when the sun was almost exactly overhead, and its rays came nearly straight down.

If I had had a glass rounded out on both sides I could have got fire any time after the sun was well up in the sky.

Now let me tell you what they call all these different kinds of glasses.