BIOMETRICS 



147 



each head fifty times and also each tail fifty times. To- 

 gether we have 100 throws of heads and 100 throws of 

 tails. To get head-head or tail-tail we have the choice 

 from 100 throws each time, while to get head-tail we have 

 the choice from 100 throws of heads plus 100 throws of 

 tails i.e., from 200 throws. Therefore the chance of 

 getting head-tail is double as great as of getting either 

 head-head or tail-tail.) Similarly, if we toss up three coins 

 we get the following possibilities : 



i H-H-H + 3 H-H-T + 3 H-T-T+i T-T-T, 



and so on with more coins. For ten coins simultaneously 

 tossed up we get the following series of possibilities 

 and their relative probabilities. 



FIG. 64. TABLE OF TEN TOSSED COINS. 



(From R. H. Lock, " Recent Progress in the Study of Variation, 

 Heredity, and Evolution.") 



The same series plotted in a curve gives us Fig. 65. 



We see here once more that the extreme possibilities are 

 rare, while the medium possibility (5 H - 5 T) is the most 

 frequent one, the other possibilities gradually becoming 

 less towards both ends of the curve. The probability 

 curve of ten coins is still rather angular, but if we construct 

 the curve for the relative probabilities of a great number 



t 



