But perhaps the question suggests itself, why take all this trouble of a circuitous route? Why not measure the distance of the sun directly, if such things can be done at all? If it is possible to measure the distance of Venus by observations, surely the sun, which has an apparent diameter thirty times as great, and which can be seen every day, and from any accessible point of the earth’s surface, gives a far ampler field for such observations. If we have instruments so delicate as to disclose to us the presence in the sun of iron, copper, zinc, aluminium, sodium, manganese, magnesium, calcium, hydrogen, and other substances, surely it will be possible to determine that comparatively gross fact—its distance from the earth. And, in truth, what becomes of the lesson we learned in our school‐ days, that the sun was just ninety‐five millions of miles away from us? And yet, strange as it may seem to those unacquainted with the subject, it has been found impossible to decide, by direct observations, the actual distance; and the distance usually accepted was not derived from such observations. As for our lately acquired knowledge of some of the constituent substances of the sun, that is derived from the spectroscope, which as yet throws no light on the question of distance. How do we ascertain the distance of bodies from us? Practice enables us to judge, and judge correctly, of the distance and size of things immediately around us almost without any consciousness of how we do it. But if we analyze the process, it will be found that we do it chiefly by using both eyes at the same time. They are separated by an interval of two and a half to three inches. As we look at an object near to us, the rays from each visible point of it must separate, in order to enter both eyes. The images thus formed on the retina of each eye differ sensibly, and we instinctively take cognizance of that difference. Speaking mathematically, the interval is a base line, at each end of which a delicate organism takes the angle of the object viewed, and our conclusion is based on our perception of the difference between them. Ordinarily, we estimate distances by the cross‐sight thus obtained. When, however, the body is so far off that the lines of light from it to the eyes become so nearly parallel that the eyes fail to perceive the minute difference between the representations formed on the retina, then we must recur to the results of past experience, and judge, as best we may, of the distance from other data than that given us at the moment by our eyesight. Thus a sailor at sea judges of the distance of a vessel on the horizon from the faintness with which he sees her; for he knows that the intervening atmosphere absorbs some of the light, so that distant objects are dim. He judges from the fact that a vessel of the form and rig of the one he is looking at is usually of a given size, and a certain distance is required to cause the