The deception at present operates to the disadvantage of the waters; for I was myself greatly deceived in the size of this pool when I looked at it from the house; and as it produces a similar effect on every person who first sees it, I must explain the causes of the deceptio visus. First. The net fence, through which the water appears, is so near the windows, that, by the laws of perspective (of which I will explain some general rules in the sequel), it acts as a false standard, and by it we measure the size of the pool. It was for this reason that I desired some cattle might be driven on the banks, which, as I have elsewhere shewn *[Castle Hill, a villa of H. Beaufoy, Esq.], are the best standard for assisting the judgment with respect to the distance, and, of course, the dimensions of other objects. Secondly. The pool is almost circular, and the eye darts round its border with such instantaneous imperceptible velocity, that it is impossible to suppose its circumference to be nearly a mile, unless we can see cattle on the opposite shores; and then, by their respective dimensions, we judge of the comparative distance. This effect, the drawing, No. XII. [our figs. 27 and 28], will elucidate, in which the sheep on one side the water appear to be larger than the cows on the other. The bay or creek may be hid by shrubs, which will give the eye a check in its circuitous progress.
