October 4, 1906] 



NA TURE 



563 



narrow groove into which light rectangular frames of 

 wood, each with a spcclacle lens in it, can be slipped and 

 will stand upright (I'igs. i, 4). I chiefly use lenses of 12, 

 24, and 48 inches ; my eye can accommodate its focus to 

 intermediate distances, but 1 possess others which are some- 

 times serviceable. Younger persons with normal eyesight 

 would want no lenses at all. The length of the box suffices 

 for cabinet-size photos. An opera-glass reversed enables it 

 to be used with larger ones, the minifying power of the 

 opera-glasses at various short distances having been ascer- 

 tained. 



Mutual mistakabilily may occur under any one or more 

 of the following conditions, whioh are to be noted, together 

 with further remarks : — 



a a. The portraits are apparently exact copies or reduc- 

 tions on different scales. 



a. They appear to be portraits of the same person at 

 about the same age, though differing in pose and dress. 



h. They would be mistaken for portraits of the same 

 person, even though they differ in sex and considerably 

 in age, if the hair had been cut and dyed alike, and the 

 dress arranged in the same way. 



*^.a 



c. As above, if much disguised, as for theatrical person- 

 ations. 



h-c. Applies to cases intermediate between h and c. 



P. Their resemblance is partial only, being confined to 

 specified features. 



The following little table saves trouble in operating ; my 

 own is more extended : — 



The procedure adopted after many trials was to measure 

 the H,„ of each portrait to the nearest half-millimetre and 

 to write it below. Then to mount the two portraits, each 

 on a separate sledge if their facial units differed, otherwise 

 on the same. When they differed, the facial unit of the one 

 about to be used for d^was distinguished as n,„, the other 

 was in brackets as («'„,). Next, after referring to the above 

 table, to send them to their respective (i^for N = s, to con- 

 sider them carefully, and to note the result. Then to do 

 the same for N=io, and so on, until the eye became 

 familiarised with the differences between the portraits. 

 Finally, guided by these provisional attempts, to fix on the 

 suitable index and letter, adding such remarks as may 

 seem wanted. 



I became gradually more consistent in judgment, as 

 ascertained bv comparing the results on different days, but 

 have felt all along that it would conduce to trustworthiiiess 

 if two or more companions worked together and criticised 

 one another, and recorded their cominon verdict. 



A very brief example will suffice. Usually an entry 

 consists of inore lines followed by general remarks. 



Two Sisters, Registers (so and so). 

 • »'». = 8-5; C«'m = 9 0) 



NO Iq2 7 VOL. 74] 



I will add a few words on dealing with mistakability 

 caused through obscurity or other hindrances to clear vision. 

 1 prepared test cards, each containing numerals printed in 

 different types, and, having ascertained by experiment the 

 value of (i, for each kind of type when just able to read it 

 in a clear light, WTOte that value boldly by its side. An 

 appropriate test card was put by the side of the portraits, 

 and at the time when the portraits themselves were just 

 mistakable, the written d^ of that row of figures which 

 were just unreadable, was noted. The value of dc remains 

 constant whatever be the character or amount of the optical 

 hindrance. If the hindrance increases, the portraits and 

 the accompanying test card must be brought nearer to the 

 eye. They will increase simultaneously in legibility. The 

 written rf,, will always show what the d,. would be in a 

 clear light. 



The applications of the process are numerous, as must 

 always be the case when a hitherto vague perception is 

 brought within the grip of numerical precision. To myself 

 it has the especial interest of enabling the departure of 

 individual features from a standard type to be expressed 

 numerically. The departure may be from a composite of 

 their race, or from a particular individual. The short- 

 comings of a pedigree animal from a highly distinguished 

 ancestor could be measured in this way. Many other 

 examples might be given. 



I must not conclude without expressing gratitude for 

 answers to a request, published by me some time ago in 

 Nature, for waste photographs from amateurs and pro- 

 fessionals. If I be allowed to mention a single name, it 

 would be that of Mr. Norman Campbell, whose photo- 

 graphs have been eminently serviceable. 



Fr.ANCIS G.4LTON. 



Models of Atoms. 

 An interesting and instructive variant of Prof. Mayer's 

 experiment with floating magnets, which has been used so 

 much to illustrate the structure of atoms, is to do away 

 with the centripetal magnetic force and to arrange that its 

 place be taken by forces arising from capillarity. This is 

 managed as follows : — 



