50 BEING AND FACULTIES OF MAN. 



point of view. Until this power is acquired all pictures 

 appear perfectly flat surfaces, while mirrored images 

 never do so. But the above will show how very accurate 

 and perfect the eye is, even with all its peculiarities, 

 none of which affects its efficiency or impairs its perfect 

 fidelity and trustworthiness, under real and ordinary 

 circumstances, even though in addition to all these 

 singularities of structure and effect we have no fixed 

 and absolute standard of measurement, lineal, superficial, 

 or solid, to apply or trust to, and are compelled to resort 

 to and adopt conventional ones, such as lineal and square 

 and cubic inches, feet, and yards, &c., or to the relative 

 size of our own hands, or other members, in reference to 

 other objects where these conventional standards are 

 wanting ; the last being a very defective standard indeed, 

 and utterly unsuitable for conventional purposes, on 

 account of the great differences in the size of human 

 hands : so that we may conceive how much inconvenience 

 and uncertainty the ancients were under if they relied on 

 the natural cubit, consisting of the forearm and extended 

 hand, until they had reduced it to an average or fixed 

 limit which excluded variation. With regard to weight, 

 again, we are in exactly the same difficulty as we have 

 just observed with regard to measure. We have no 

 positive and abstract standard of the weight of bodies. 

 Their weight is matter of comparison only, and relative 

 to themselves inter se or to us. What feels heavy to us is 

 the primary standard, but is incapable of being made a 

 conventional one, from the difference between each man's 

 strength and that of his neighbour. And here, again, for 

 general purposes we are compelled to adopt a conventional 

 standard on which all are agreed, such as ounces, pounds, 

 or the cubic foot of water as unity, &c. ; and even these do 

 not afford us a uniformly fixed accuracy, for every kind of 

 weight varies according to its altitude or distance from 



