of iJie Kye in relation to the Perception of Uititance, 51 



ononnous iniportance in the estimation of distances in general ; 

 for there can be no doubt that where these means are avail- 

 able, the mind is guided to a large extent by them in the for- 

 mation of an estimate of distance, unless carefully trained to 

 disregard them and to depend on other circumstances, or 

 unless, as may happen to individuals, some physical injury 

 has occurred to render the eye abnormal. With respect to 

 the consideration advanced in (c), I have some reason to 

 doubt whether we really do judge apparent distance by the 

 muscular sensations of the external and internal recti muscles 

 of the eye. Stereoscopic pictures appear to me equally ^' solid '* 

 whether in a Wheatstone's reflecting stereoscope, a Brewster's 

 lenticular with prism-shaped lenses, or a Helmholtz's with 

 parallel axes, or by superimposition by effort of the recti 

 muscles, in each of which cases a different convergence is 

 required. Moreover it would be very inconvenient if our 

 estimate of distance depended on the action of these muscles, 

 as they are peculiarly liable to fatigue. 



14. Retinal magnitude, to use the term adopted by Wheat- 

 stone, whether regarded in its muscular {d) or its optical (<?) 

 sense, affords only an indirect means of estimating distance 

 by association with ideas previously acquired concerning the 

 retinal dimensions of objects of known size at known distances. 

 Nothing is more easy than deception arising from this cause. 

 We constantly hear the magnification of the telescope spoken 

 of as though the enlargement of the retinal image produced 

 the same effect as if the object were brought so near to the 

 eye as to yield a retinal image of equal area. This is evidently 

 not the case, as the distant object, however much magnified in 

 the telescopic image, is still seen by rays travelling in paths 

 nearly parallel to each other, and not containing angles as 

 great as those contained by rays proceeding from an object 

 really near. Hence a telescopic presentation of an object is 

 flatter than the object itself appears when brought sufficiently 

 near to give a retinal image of equal magnitude. 



15. The fallacy of judging distances by apparent magnitude 

 is made evident by the absurd comparisons often made between 

 the sun or moon and other objects, some individuals compa- 

 ring them to the size of a coin, a plate, or a cart-wheel. 

 Comparatively few persons could at once tell you correctly 

 which appears the larger, the setting sun or a threepenny- 

 piece held at arm's length — the fact being that the angular 

 magnitude of the latter exceeds that of the former. 



16. The following experiment* illustrates a remarkable 



* My attention was first drawn to tliis curious fact by Mr. Joseph 

 Beck, of the emineni; firm of Smith and Beck. 



E 2 



