184 



PROGRESS IN MICROSCOPY 



To preclude any errors in locating the tints, the following procedure 

 is suitable: the polarizers are crossed and the microscope adjusted so 

 that a dark field obtains (J = 0), outside A[ and A',. The adjustment 

 is continuously altered (leaving the polarizers alone) so as to show 

 the colours sequentially until the field exhibits the purple of the first 

 order (zl = c = 0-565 /j). Since the colours may be followed along 

 Newton's scale of colours, no error need be expected in locating c. 

 In this adjustment the colour is denoted by a at A[ and b at AL Now 

 the reverse procedure will enable one to locate these two colours. 

 The microscope is adjusted, varying A so that the path diiference 

 A — (n' — n) is zero at A[ which is now very dark. Referring once more 

 to Newton's scale of colours while carrying out this operation will 

 determine and, therefore, identify the initial colour at A[, i.e. when 

 c = 0-565 ju on all sides of A[ and A!^. Hence the colour surrounding 

 A[ and at A[ are definitely determined: 



(1) Purple (first order) on all sides of A[, 



(2) Colour correlated to the value e at A[. 



n > n 



J — L 



b > 



n > n 



Fig. 7.4. First arrangement of the two 

 wave surfaces. 



b > a 



•^9 



Fig. 7.5. Second arrangement of the two 

 wave surfaces. 



In fact, these two measurements are adequate since they evince c 

 and a, i.e. (n'—n)e, according to the equation (7.4). There is no need, 

 therefore, to determine the colour and, hence, the path difference at AL 

 However, this may be done as a test. Having restored the first-order 



■^2 



■2", 



n > n 



a>b 



n > n 



a>b 



Fig. 7.6. Third arrangement of the two 

 wave surfaces. 



Fig. 7.7. Fourth arrangement of the two 

 wave surfaces. 



purple around A[ and A'^ (c = 0565 /<) the colour of the image ^o is 

 defined by h. Exactly the same operation is then resumed : :1 is varied 

 so that black is developed in the image AL The colours being followed 

 along Newton's scale of colours, it is possible to determine the initial 



