THE EYE. 



noon are by measure the same, when they are by estimation, and 

 by the irresistible evidence of sense, so extremely different ? This 

 is explained, not by an error of the sense, for there is none, but by 

 an erroneous application of those means of judging- or estimating 

 distance which in ordinary cases supply true and just conclusion!;;. 



When the disc of the sun is near the horizon, a number of 

 intervening objects of known magnitude and known relative 

 distances supply the means of spacing and measuring a part at 

 least of the distance between the eye and the sun ; but when the 

 sun is in the meridian, no such objects intervene. The mind, 

 therefore, assigns a greater magnitude to the distance, a part of 

 which it has the means of measuring, than to the distance no part 

 of which it can measure ; and accordingly an impression is pro- 

 duced, that the sun at setting is at a much greater real distanc2 

 than the sun in the meridian ; and since its apparent magnitud3 

 in both cases is the same, its real magnitude must be just so much 

 greater as its estimated distance is greater. The judgment, 

 therefore, and not the eye, assigns this erroneous magnitude to 

 the disc of the sun. 



It is true that we are not conscious of this mental operation. 

 But this unconsciousness is explained by the effect of habit, 

 which causes innumerable other operations of the reason to pass 

 unobserved. 



81. As the eye forms no immediate perception of distance, 

 neither does it of form or of magnitude, since, as has been already 

 proved, objects of very different real magnitudes have the same 

 apparent magnitude to the eye, of which a striking example is 

 afforded in the case of the sun and moon. Nevertheless, although 

 the eye supplies no immediate perception of the real magnitude of 

 objects, habit and experience enable us to form estimates more or 

 less exact of these magnitudes by the comparison of different 

 effects produced by sight and touch. 



Thus, for example, if two objects be seen at the same distance 

 from the eye, the real magnitude of one of which is known, that 

 of the other can be immediately inferred, since, in this case, the 

 apparent magnitudes will be proportional to the real magnitudes. 

 Thus, for example, if we see the figure of a man standing beside a 

 tree, we form an estimate of the height of the latter, that of the 

 former being known or assumed. Ascribing to the individual 

 seen near the tree the average height of the human figure, and 

 comparing the apparent height of the tree with his apparent 

 height, we form an estimate of the height of the tree. 



82. It is by this kind of inference that buildings constructed 

 upon a scale greatly exceeding common dimensions are estimated, 

 and rendered apparent in pictorial representations of them 



88 





