DETERMINATION OF THE AXES OF ELASTICITY. 



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bined colours, to just the same extent as the latter do in regard 

 to the acceleration colour. Thus, if we combine blue II. with 

 yellow I., we get red in the one position, which a skilful observer 

 will recognize as red II. ; in the other position a yellow, which 

 agrees with yellow I. In our scale of colours also red II. and 

 yellow I. are actually about equidistant from blue II., and a 

 repetition of the combination with known selenite plates affords 

 a perfect proof that blue II. and yellow I. give the above- 

 mentioned colours in the positions of acceleration and retardation. 



Since this repetition of the combination always affords the best 

 check, we have appended two tables, in which the colours of 

 acceleration and retardation are collected together for a series 

 of cases. For the first three orders their names are based upon 

 their classification into six colours each, whose distances upon a 

 polished crystal wedge are about equal. If we therefore denote 

 the colours of the first series by 1 6, those of the second by 

 7 12, and so on, we can pre-deterinine any combination colour 

 by addition and subtraction of the corresponding numbers, 

 whereby rapid and accurate interpretation of the observed colour 

 is considerably facilitated. The colours 6 (red I.) and 3 

 (white I.) give, for instance, in the position of acceleration 

 6 + 3 = 9 (green II.) ; and in the position of retardation 

 6 3 = 3 (white I.). We notice, moreover, that the colours 

 of the selenite plates of commerce do not always agree exactly 

 with the theoretical gradations, and that the same numbers of 

 the same maker often produce a sensibly different tint. 



The second table is of special practical value from the fact 

 that a selenite plate, which gives red I. as a rule, most easily 

 admits of the recognition of the rise and fall of the colour. 



TABLE I. 



z 2 



