it makes upon the retina, just as its apparent distance from the observer is determined chiefly by the distinctness of the impression formed upon the background of the eye. The rays of light reflected from the distant mountain made a distinct image upon my retina) because they traversed a rarefied atmosphere of uniform density which produced the minimum amount of refraction, dispersion, and absorption. Previous to this time I had been accustomed, under Eastern skies, to view distant objects through media neither so rare nor so uniform as this mountain air, and it was not, therefore, strange that my calculations of distance should in this case be erroneous. Such phenomena, familiar enough to most travelers and to every dweller in the cool, thin atmosphere of mountainous regions, are almost startling when seen for the first time. It is difficult to believe that the huge, stony mass, apparently 
Fig. 1. so near—certainly so plainly seen—is over half a hundred miles away.
The illusion as to distance does not, however, extend to the matter of size. Mountains and hills may, under certain atmospheric conditions, appear to be near at hand when they are actually far away, but their apparent size remains always the same. The same mountain would appear of just the same size in Colorado as in Vermont. We know this because objects equally distant and of the same size always subtend the same visual angle. The greater the distance from the eye, the smaller the visual angle and retinal image; the less the distance, the greater the angle and the larger the image—as the following diagram (Fig. 1) shows:
The rays of light falling through the pupil upon the retina, b m c, cross at the nodal point a. The near object, 7 8, subtends a larger visual angle, 7 a 8, and makes a larger retinal image, 5 6, than the distant object, 1 2.
It would be interesting to test the truth of these statements by actual experiment, if ideas of size and distance did not, unfortunately, vary with the individual, and were not subject to almost daily modification by experience and other influences. Calculations as to the actual size and distance of the most familiar objects are, within certain limits, but pure guesses on the part of the great majority of people; so that, even if it were worth the while, the most of us could never become experienced enough,
