606 J. W. EVANS ON THE DETERMINATION OF MINERALS UNDER 



retardation, which is usually approximately the same for all 

 the colours of the spectrum. 



Within certain limits every amount of relative retardation is 

 distinguished by its own characteristic interference tint between 

 crossed nicols, and these tints are practically the same for the 

 majority of minerals, though the thickness required to give rise 

 to a particular colour varies greatly for different minerals and 

 according to the direction in which the same mineral may be 

 cut. It is only in those minerals in which the relative retardation 

 varies for different colours that unusual or anomalous colours are 

 seen. These minerals are so few in number that the occurrence 

 of their characteristic anomalous colours furnishes a ready means 

 of distinguishing them. The indigo- blue seen in many thin 

 sections of chlorite is a familiar example. 



The normal interference colours commence with complete 

 darkness at zero relative retardation and pass through grey, 

 white, yellow, orange, and red, at the end of which the relative 

 retardation reaches 550 micro- millimetres. These constitute the 

 colours of the first order. Then follow purple, violet, blue, 

 green, yellow and red up to a relative retardation of 1,100. These 

 are the colours of the second order. Every addition of 550 micro- 

 millimetres corresponds to another order with a similar suc- 

 cession of colours, which gradually become more complex till 

 they are only represented by delicate shades of green and pink, 

 and with a relative retardation of about 4,000 micro-millimetres 

 they slowly pass into white light, the ' white of the higher 

 orders." The colours are said to be lower or higher according 

 as they result from a less or greater amount of relative retar- 

 dation. 



If one nicol be rotated through a quarter turn so that the 

 directions of vibration of the two nicols are parallel, the com- 

 plementary colours are seen, which commence with white and 

 pass through brown, red and blue to the yellowish green that 

 marks the end of the first order at 550. The second order passes 

 through yellow, red and blue to green again, and in the higher 

 orders the colours gradually fade away through pinks and greens 

 into white light exactly as with crossed nicols. 



The amount of the relative retardation in a crystal section 

 may often be roughly estimated directly from the interference 

 colours between crossed and parallel nicols by comparison with 



