But it can hardly be maintained that any and all the liberties which a painter or a whole school of painters choose to take with fact in their presentation of Nature--are beyond criticism. It is possible for a landscape painter to improve in his treatment of the moon by better observation and increased knowledge--just as other painters have learnt not to introduce into their pictures the sort of wooden rocking-horse to stand for a beautiful living animal, which satisfied Velasquez, Carl Vernet and the ancient Egyptians. FOOTNOTES: [Footnote 1: See note on page 46.] [Footnote 2: "La Representation du Galop dans l'art ancien et moderne," 'Revue Archeologique,' vol. XXXVI _et seq._, 1900.] [Footnote 3: A word is needed in amplification of what was said on p. 26 as to the blending of successive images produced on the retina of the eye by the bioscope or cinematograph or by the old "wheel of life." The point which is of importance is not the length of time during which the stimulation of the retina caused by an image _endures_--becoming weaker and weaker as fractions of a second pass--but it is this: How long will a stimulus last in _undiminished brightness_? How soon must it be followed by another stimulus (another image) so that there may be fusion or continuity, the one succeeding the other before the earlier has had time, not to disappear, but to decline. If it has had time to decline in intensity, the appearance of flickering results. That is what the cinematographer has to avoid. It is found that a quicker succession--a shorter interval--is necessary with strong light than with weaker light in order to produce continuity. With a faint light the interval may be as great as one-tenth of a second; with a strong light it must not exceed one-thirtieth (or with still stronger light, one-sixtieth) of a second. With the stronger light there is a more rapid and a greater loss of the initial intensity of the impression or effect of stimulus, and though each successive effect remains as long, or longer, in dwindling intensity, you get want of continuity, or "flicker."] [Footnote 4: What we may call the "visual size" of the sun happens to be owing to its far greater size and its far greater distance from us--very nearly the same as that of the moon--and is subject to the same numerical law of apparent diameter, viz. a disc of any given measurement in diameter will cover it exactly when held at a distance from the eye which is 115 times that measurement.] CHAPTER V THE JEWEL IN THE TOAD'S HEAD To what jewel or precious stone was Shakespeare alluding when he makes the exiled Duke in "As You Like It" (after praising his rough life in the forest of Arden, and declaring that adversity has its compensations), exclaim: "The toad, ugly and venomous, Wears yet a precious jewel in his head"?