POPULAR FALLACIES. 



91 



pared with the same object seen on the meridian ? Yet nothing is more easy 

 than to prove, as a matter of fact, that these impressions are fallacious. Let 

 any one adopt any convenient method which may occur to him, to measure the 

 apparent magnitude of the sun in the horizon, and again in the meridian, and 

 he will find them the same. This may be accomplished by extending two 

 threads of fine silk parallel to each other in a frame, and placing them in such 

 a position, and at such a distance from the eye, that when presented to the sun 

 or moon, in the horizon, they will, exactly, touch its upper and lower limb, so 

 that their apparent distance asunder will be equal to the apparent diameter of 

 the lunar or solar disk. 



If this arrangement be preserved, and the sun or moon be viewed in the 

 same manner when at, or near, the meridian, it will be found that the threads 

 will equally touch its upper and lower limbs, and that their interval will still 

 measure its apparent diameter. 



In fact, all astronomical telescopes are provided with an apparatus by 

 which observations of this kind can be made with the greatest accuracy and 

 facility. There is a system of parallel fibres or wires extended across the field 

 of view, which are removed toward or from each other by an adjusting screw. 

 The magnitudes of the disks of the sun, moon, or planets, can be ascertained 

 by moving two of these wires until one of them shall touch the upper, and the 

 other the lower limb of the disk. By means of such an instrument, the mag- 

 nitude of the sun or moon in the horizon, and in the meridian, may be meas- 

 ured, and it is found not to be sensibly different. 



It will, therefore, be evident that whatever be the cause of the illusion, the 

 apparent magnitude of the sun or moon is not greater at rising or setting than 

 in the meridian. Whence, then, it may be asked, arises an impression so uni- 

 versally entertained ? In fact, the moon is 4,000 miles further from us when it 

 sets or rises, than when it south's, or passes the meridian, and, strictly speaking, 

 therefore, its apparent diameter, instead of appearing larger, ought to appear 

 about a sixtieth part less. 



This illusion has been attempted to be explained by supposing that, as the 

 moon is less brilliant in the horizon than in the zenith, we open the pupil of 

 the eye wider on looking at it when in the horizon, and it is for this reason we 

 see it larger. But this reasoning is, obviously, invalid, inasmuch as we know 

 from the principles of optics, that the image produced in the eye has the same 

 magnitude to whatever extent the pupil may be dilated or contracted. 



The explanation of this singular effect, in which all astronomers appear to 

 concur, refers it to mental, and not optical causes ; strictly speaking, it is not 

 an optical illusion. The organ of vision does not, itself, present to us a larger 

 moon in the horizon than in the zenith, as is proved incontestably by the mi- 

 trometric wires. The error is, then' one of the mind and not one of the sen- 

 ses. The estimate which we form of the actual magnitude of any visible ob- 

 ject, depends on a comparison of the apparent magnitude which that object 

 presents to the eye, with the distance at which we imagine it to be. Thus if 

 there be two objects, buildings, for example, which have to the eye the same 

 apparent height, but which we know or believe to be at different distances from 

 us, we instinctively, and without any operation of the judgment, of which we 

 are conscious, conceive that which is more distant to be the largest ; and in 

 like manner, if two objects, which are at different distances, appear to the eye 

 to be of different heights, the more remote being less than the nearer, we judge 

 them, nevertheless, to be equal in size, ascribing, by an unconscious action of 

 the mind, the difference of their apparent magnitudes to their difference of dis- 

 tance. 



To apply this reasoning to the case of the sun or moon, we are to consider < 



