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along GJ: and another little mirror at J sends them into the eye A. Rays which start along FD are similarly reflected along HKB into the eye B. Hence without moving his own eyes at all, the observer measures the "squint" of the wide artificial eyes C and D by turning the screw (or both screws, if there are two). He then knows the distance of the enemy and tells his comrades for what "range" to sight their rifles. If they did not find out the "range" in this way, the bullets would either go over the heads of the enemy, or hit the ground in front of them.[1]

Now the astronomer is fully alive to this method of getting over the great difficulty. What it comes to is that we make the base as long as possible. Nature has put our eyes so close together that the base is very short: and though it suffices for estimating the distance of a cigar tip, or of people in the same room, it is too short for telling us the distance of the enemy half-a-mile away. So the range-finder is made with a much longer base and the difficulty is largely reduced. But for greater distances still, such as that of the Moon, the range-finder becomes as useless as our eyes: the base is not nearly large enough. Instead of having our artificial eyes a few feet apart, we must put them miles apart; and we may as well put them as far apart as we can, let us say, on opposite sides of the Earth (Fig. 13); and then we find that the "squint" required to set both eyes (that is to

  1. Since these lectures were delivered, the use of the periscope has become familiar to most of us. The eyes on horns could be represented by using a periscope for each eye.