WAYSIDE OPTICS. 



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I 



it makes upon the retina, jnst as its apparent distance from the 

 observer is determined chiefly by the distinctness of the impres- 

 sion formed upon the background of the eye. The rays of hght 

 reflected from the distant mountain made a distinct image upon 

 my retina) because they traversed a rarefied atmosphere of uni- 

 form density which produced the minimum amount of refraction, 

 dispersion, and absorption. Previous to this time I had been ac- 

 customed, 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, ap- 

 parently 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 al- 

 ways 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, sub- 

 tends 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, unfortu- 

 nately, vary with the individual, and were not subject to almost 

 daily modification by experience and other influences. Calcula- 

 tions as to the actual size and distance of the most familiar ob- 

 jects 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, 



