136 ELEMENTS OF APPLIED MICROSCOPY. 
of the ridges and furrows characteristic of that digit, a 
record necessarily free from the error which may attend 
ordinary anthropometric measurements. Galton has 
shown that the pattern of the skin persists unchanged 
through life and is unaltered in its essentials by cuts, 
burns, or any ordinary accidents. The variations are so 
great that he calculated the chances of identity between 
two single prints to be only one in sixty-four billion. 
When ten digits are compared, identification is absolute. 
Such records, in order to be of.any practical service, 
must be readily arranged and classified; this Galton 
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Fic. 51.—FINGER-PRINT PATTERNS. (Redrawn after Galton.) 
has done so successfully that, out of a “‘finger-print direc- 
tory” of 2632 persons, any pattern could be located in 
three minutes. In the first place, the patterns of the skin 
may be grouped under three heads. In all cases the 
papillary ridges run across the fingers in the vicinity 
of the third joint, and at the tip they follow the curve 
of the nail in a rounded arch. Sometimes the ridges 
between follow the outer ones in a more or less even 
arch of lessening convexity; this is the arch type (1, Fig. 51). 
Sometimes the intermediate ridges form a loop running 
from one side inward to the center of the bulb and then 
doubling back again. Obviously at the opposite side 
