. Our senses series; . gh it casts the sameimage on the retina that it does at the horizon, wethink it much smaller than at the horizon. When,however, we see the moon at the horizon, themany Intervening objects make us think it is muchfarther away than at the zenith, and therefore wesee it as much bigger as we think it is far-ther away. This illusion can be dispelled byrolling a paper tube and looking through it at themoon on the horizon. All intervening and sur-rounding objects being cut off from our view, themoon will come down to a ten cent piece in appar- 88 THE SENSE OF SIGHT ent size. Th
. Our senses series; . gh it casts the sameimage on the retina that it does at the horizon, wethink it much smaller than at the horizon. When,however, we see the moon at the horizon, themany Intervening objects make us think it is muchfarther away than at the zenith, and therefore wesee it as much bigger as we think it is far-ther away. This illusion can be dispelled byrolling a paper tube and looking through it at themoon on the horizon. All intervening and sur-rounding objects being cut off from our view, themoon will come down to a ten cent piece in appar- 88 THE SENSE OF SIGHT ent size. The above cases of the moon are typi-cal illusions due to wrong estimation of wrong estimate is a primary illusion, ex-plained by the lack of eye muscular feeling inlooking through empty space and hence underesti-mation of the distance, or greater muscular exer-tion in looking through filled space and hence over-estimation of the distance. The wrong estimateof the distance of an object, however, affects our. Fig. 20. judgment of the size of the object, and this is asecondary illusion, an illusion due to a wrong judg-ment. The reader may be helped in understandingthese illusions if he will carefully study Fig. 20,which for simplicity is diagrammed for only oneeye. Suppose that CD be an object, a house forinstance, on a Colorado mountain side, seen acrossthe valley from another mountain side. Owing to VISUAL ILLUSIONS 89 the clear air of that dimate, which makes thehouse look very distinct, and to the lack of inter-vening objects, the distance away of CD is gener-ally underestimated, hence it is thought to be atAB. Now it will be readily seen that if CD wereat AB, as we think it is, it would have to be assmall as AB to still cast the same image on theretina as it does where it is. It is really, how-ever, at CD and casts the same image on theretina as if it were reduced to the size and dis-tance of AB, therefore it appears to be at versa, if we think an object
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