PHychoUxjical InvestujatioiM 227 



in the pitch of a musical note, presumably deeigned entirely by Darwin him- 

 self". All these instruments are very simple in character and ought not to be 

 overlooked by the iuithropoiiietrician. In particular the chronograph is very 

 ingenious. Darwin thus de.scril)es it: 



"A wmxien rtxl is supported at its upper end by a detent, and can be released at will. The 

 rod tli(>n fulls frei'ly in space piissinj; tlirougii a hnlo in a fixed diaplini^ni. A weijjht in the 

 form of u ring larger than the hole in the diaphragiii, rests on a collar near the top of the rod. 

 Thus, aft<'r rod and weight together have fallen a definite distance, the weight is caught by the 

 diaphragm and makes the signal sound, while the rod still continues to fail. On hearing the 

 signal sound the person to lie teste<l presses down a lever, thereby releasing a spring clamp 

 which grips the falling rod finnly. The interval of time lietween the signal sound and this 

 operation is measured by the space the rtxl has fallen through, and is read at once in hundredths 

 of a second from graduations on the rod itself." (p. 9.) 



Liistly, we may note that early in 1890 the Royal Society appointed a 

 committee of wliich I^ord liayleigh wius Chah'man, Captain Abney, Secretary, 

 and Sir George G. Stokes, then President of the Society, Brudenell Carter, 

 Church, Evans, Micliael Foster, Dr Farquharson, Galton and W. Pole were 

 members. The average attendance was six to eight and Galton appears to 

 liave attended with tne greatest regularity. At the fifth meeting he pre- 

 sented a memorandum fis to testing colour blindness, (i) "under not di.ssimilar 

 conditions to those in whicli signals are seen by sailors and engine drivers," 

 (ii) in which the attendant does not know the colour being exhibited, and 

 (iii) the subject indicates the colour not by its name, but by turning a 

 thumb and finger piece attached to each colour box, with rough side up for 

 red, smooth side up for green, and into an intermediate position for neutral 

 tinted colour. The whole takes place in the dark, and the subject's answers 

 are ascertained by examining the colour-boxes later. There were to be nine 

 colour-boxes, three for red, three for green, three for neutral tint; each 

 series with one, two and three thicknesses of gla-ss. A good many other 

 methods of measuring colour sense were described, and nmcli evidence taken. 

 I do not know whether Galton's apparatus ever came into practical use : like 

 all his instruments, it was very simple, the light being provided by a police- 

 man's "bull's eye." 



Galton's investigation of mental characters led hira directly to the 

 Weber- Fechner Law of the geometrical mean. Such a law appears directly 

 opposed to the Gaussian hypothesis that the arithmetic mean gives the best 

 " medium," i.e. the most probable or modal value of a series of ol)servations. 

 Galton accordingly proposed the following problem : Assuming the geo- 

 metrical mean and not tiie arithmetical n»ean to give the best "medium," 

 what is the mathematical form of the frequency distributions'? Galton seems 

 to have held that not only in tint and length judgments', but in many 



' K. Soc. I'roc. Vol. XXIX, pp. SC.'i-C, 1879. "The Geometric Mean in Vital and Socia 

 Statistics." 



" "Three rods" (ialton writes "of lengths a, h, c if taken successively in the hand appear 

 to difTcr by equal intervals when a:b::b:c and not when a - 6 = 6 - <•" (p. 366). I have made a 

 number of individual t<.'sta on myself, but my judgment supports an arithmetic not a geometric 

 mean in the case of the three rocls. I once asked between 200 and .300 individuals to select on 



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