noon
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 could n'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 .