52

A VOYAGE IN SPACE

off at 6 feet, 12 feet, 18 feet, and so on, from A: and you will be able to see on which of these the light falls. But I want one of my young friends to turn his back on the screen so that he cannot see where the light falls: all he can now see is the amount of "squint" marked on this card, and yet you will find that by reading the "squint" he can tell you where the light is falling. Thus I point the telescope and from the "squint" he tells you that the squint is marked 1, meaning that the distance is 12 feet: we change it and he tells you that the "squint" is 1½, that means 1½ times the base, or 18 feet; change it again and he says it is between ½ and 1; or between 6 feet and 12 feet, as we see it is. He can tell just as well what the distance is from the "squint," as we can by seeing it on the screen. And this is no conjuring trick: it is the simplest and most straightforward process: and it is the same process which the astronomer uses to find the distance of the Moon.

But before going on to the Moon, let us consider this card of "squint" a little (Fig. 10). We put 1 for a "squint" on an object just as far away as the base. In our case the base was 12 feet; but the same card would serve if the base were a mile long: when the "squint" was 1, the distance would then be a mile. The mark 2 tells us the "squint" corresponding to twice the base: 3 to 3 times the base, and so on. We can also mark ½ for half the base; or other fractions. But what I want you to note is that all the marks for distances greater than 1 fall in one half of the arc: they all fall between E and Z: in the part AE the marks are all for distances less than 1. Moreover, the distance 2 uses up a considerable part