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50 eyes generally unconscious, but it is also vague: that is to say, we seldom express the distances in feet and inches: we usually judge whether one thing is nearer than another, or farther away, or at the same distance. Thus, when I try to light a cigar, I have to use my eyes to tell me when the match is at the same distance as the end of the cigar: for if I put the match nearer I shall spoil the cigar by lighting it in the middle; and if I put the match farther away than the end I shall not get a light at all. This equality of distance is gauged by squinting with the eyes; and if you look at your father's eyes when he is lighting a short cigar, or a short stump of one, you will see him squint horribly. But if he has a very long cigar like this, he will squint far less. For that reason it is not quite so easy to light a big cigar. If I could get one as long as this room, even if my arms were long enough to put the match near the end, I might not hit the end without some trouble: for the amount of squint would be so small that I could scarcely feel it, and it would be very nearly the same even if I put the match a foot away from the cigar end.

Now astronomers use the same kind of method, and meet with the same difficulties, when they measure the distance to the Moon. Instead of two eyes, they use two telescopes, one at a place we will call A, the other at a place B; and they measure the distance AB, which is called the "base," and is of great importance (we do not need to measure the distance between our two eyes, which is the "base" for ordinary life, because we are thoroughly familiar