118 



DISCOVERY 



satellites in this connection, a misunderstanding which 

 is often repeated even in textbooks, though I clearly 

 indicated the accurate way of regarding the matter 

 in my address to Section A of the British Association 

 at Cardiff in 1891, and although Professor Tait in his 

 treatise on Light had expounded it before. To illus- 

 trate the popular misconception it will suffice to quote 

 from a quite recent article by Canon Edmund McClure, 

 in The Beacon, called " The Ether and the New 

 Physics," in which he speaks as follows about eclipses 

 of Jupiter's satellites : 



" The Danish astronomer Roemer found in 1675 that 

 these eclipses were observed some 22 minutes earlier 

 when the earth was at one point of its orbit than six 

 months later, when it was farther away by the orbit's 

 diameter, that is by 186,000,000 miles. Hence the 

 light reflected from the satellite at the instant before 

 its eclipse took 22 minutes to traverse the diameter 

 of the earth's orbit. More accurate observations 

 reduced the 22 minutes to 16, and it was an easy cal- 

 culation to deduce from this that light was not in- 

 stantaneous, as had been supposed, but travelled at 

 the rate of 186,000 miles a second, a velocity confirmed 

 by later experiments." 



What Canon McClure no doubt means is that the 

 accumulated discrepancies during recession or approach 

 amounted in the end to the 16 minutes he speaks of. 

 And that is one way of expressing the result : but it is 

 not a clear and instructive way. It verj' often happens 

 that the accurate method of regarding a phenomenon 

 is really better and clearer, and always more instruc- 

 tive, than something modified to suit what is thought 

 to be ordinary apprehension : and hence it seemed 

 worth while to call attention to an improved mode 

 of statement, which is more in accordance with the 

 actual facts. 



been obtained which corroborate and amplify the 

 experience of the pioneer workers on the subject. 



Evidence is still being accumulated to show that the 

 patterns are permanent throughout life. The late 

 Sir William Herschel had a thumb-print made in 1859, 

 and one made in 1914 was found to be identical with 

 it. It will be long before his record of fifty-five years 

 is beaten. A striking illustration of the indestructi- 

 bility of the patterns, to which Dr. Faulds was the first 

 to call attention, has recently been given by Mr. B. 

 Wentworth, of New Dover, U.S.A., who has sent me 

 a series of prints showing the effect of a burn upon the 

 pattern and the gradual reproduction of the original 

 design with the healing of the injury. 



During the last few years I have made an extended 

 series of experiments to ascertain after what intervals 

 of time it would be possible to develop a latent imprint 

 of a finger upon paper, such as the leaf of a book. 

 For this purpose I have studied the various methods 

 which have been suggested for the purpose and have 

 devised several new ones (see Analyst, 1920, 45, 122). 



Up to that time these methods had been mainly 

 mechanical in their action ; and my results indicated 

 that their efficiency would largely depend upon the 

 absorptive capacity of the surface of the material 

 upon which the print had been made. For example, 

 if a print has been left upon glass or upon the surface 

 of a japanned tin box, it can be developed even after 

 some years by dusting it with a suitable powder, such 

 as a mixture of chalk and mercury or powdered graphite 

 (as in the official English method), with lycopodium 

 powder coloured with an aniline dje, or with heated red 

 lead or lead iodide. Prints on highly sized paper 

 can be developed in this way after some weeks, but 

 those on ordinary paper will only give a poor result 

 after a week or two, and imprints on blotting-paper, 

 although they can be sharply developed for some hours 

 after being made, will only show a faint smudge after 

 about a week. 



Recent Work on the 



Ridge Patterns of the 



Skin 



C. A. Mitchell, M.A., F.I.C. 



The general principles upon which identification by 

 means of finger-prints is based are now so well known 

 that most people are acquainted with the methods of 

 classifying the patterns and of using them in the 

 detection of crime. There are, however, certain new- 

 developments of the methods which are, as yet, but 

 little known, and many interesting results have also 



Methods of Developing Latent Imprints 



The use of a liquid reagent was first developed by 

 Forgeot in 1891, and after various experiments he 

 found ordinary writing ink to be the most suitable 

 for the purpose. The principle upon which its use 

 is based is that there is normally a slight secretion of 

 oil in the fingers, and that this is conveyed to the parts 

 of the paper with which the ridges come in contact. 

 The marking made by the ridges will therefore repel 

 the ink, whereas the intermediate furrows and the rest 

 of the paper will be stained. Prints made in this way 

 are therefore "negatives," whereas those made by 

 the mechanical method of adhering powder are " posi- 

 tive," the ridge receiving most of the pigment and the 

 furrows remaining relatively uncoloured. 



