noon
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 . Now let me tell you what they call all these different kinds of glasses . One that is flat on one side and bulges out on the other is called a _ convex lens _ ; if it bulges out on both sides it is a _ double convex lens _ ; if it is hollowed in on one side and flat on the other it is a _ concave lens _ ; if hollowed in on both sides we call it a _ double concave lens _ ; and when it is hollowed in on one side and bulged out on the other , as any watch crystal does , it is a _ concave convex lens _ . ''