346 PART V. WORKING STAGE. 



symphony; the place of diagrammatic curves in statistical and 

 other studies ; and countless other problems including the bulk 

 of the facts of geology and physiography, the steady growth 

 and further development of the arts, crafts, sciences, and 

 social institutions, and everything capable of being treated 

 quantitatively, need to be classified under this head. 



In this way, by using the stereoscope with sets of photographs, 

 taken from half-an-inch to ten inches apart, we convincingly 

 demonstrate that our sense of the third dimension is due, in 

 part at least, to the distance between the eyes. Thus "Sir 

 H. Davy, from finding that the flame of hydrogen gas was not 

 communicated through a long slender tube, conjectured that 

 a shorter but slenderer tube would answer the same purpose; 

 this led him to try the experiments, in which, by continually 

 shortening the tube, and at the same time lessening its bore, 

 he arrived at last at the wire-gauze of his safety lamp". 

 (Whately, Logic, pp. 237-238.) 



Again. Warm water at rest in an open vessel lacks deci- 

 dedly the character of a solid; yet "the water a foot or so 

 away from the fireman's hose, may be struck with a hammer, 

 and the latter will rebound as though from an anvil". Simi- 

 larly, chromium added to steel in a certain proportion renders 

 the latter rustless, and the best incandescent mantle contains 

 a mixture of 99 per cent, of thorium oxide and 1 per cent, of 

 cerium oxide. To borrow an example from Venn: "If we are 

 shown two glasses of water, one from the sea, and one from 

 the Lake of Geneva, no one can detect any difference in their 

 colour. But let us have enough of each in vessels side by 

 side, and any eye could detect the degree and nature of the 

 contrast." (Logic, p. 536.) So, too, if we place two ounce 

 weights side by side, no mutual gravitational effect is observ- 

 able; but a pendulum near a mountain is deflected sensibly. 

 Thus snowflakes appear white in masses and transparent when 

 detached, and the corpuscles of the blood seem red when in 

 numbers and somewhat yellow separately. Anthropology offers 

 us a pertinent illustration of some consequence. Human beings 

 are as a rule strictly divided into white, yellow, and black ; yet 

 not only does Prof. Tyler in his Anthropology (p. 67) inform 

 us that "on the whole it seems that the distinction of colour, from 

 the fairest Englishman to the darkest African, has no hard and 

 fast lines, but varies gradually from one tint to another", 

 but the facts appear rather to bear witness to a single colour 

 from the palest to the shadiest yellow. Darwin applies this 

 Conclusion perhaps more frequently than any other: "This 

 feather-mark [the ocellus on the tail-coverts of the peacock] 

 was properly considered a serious difficulty to Darwin's theory 

 because of its remarkable character. But with consummate in- 

 genuity he undertook to connect it by a series of less and less 

 remarkable markings with the ordinary feather-markings of the 



