THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 605 



of vibration in the crystal section that is to say, to ascertain 

 which of these is the direction of vibration of light with the 

 greater velocity, and which that of light with the less velocity. 

 The character of the former direction of vibration or extinction 

 is said to be fast ( ) and that of the latter slow ( -j- ). 



Relative Retardation. We can also determine at the same 

 time the amount of the relative retardation in other words, the 

 distance by which the slow- moving vibrations have lagged 

 behind the faster. Both are delayed in traversing the section, 

 but the former more than the latter. Relative retardation is 

 usually measured in micro-millimetres or millionths of a milli- 

 metre. The character of any definite direction in a crystal 

 section, e.^. t one of its longer sides, is also said to be fast, or slow, 

 according as it coincides or makes an angle of less than 45 

 with the fast, or the slow, directions of vibration, and to be 

 neutral when it bisects the angle between them. 



For the purpose of making these determinations the section is 

 brought into a position of extinction and then the stage rotated 

 through 45, so that the directions of vibration in the section are 

 diagonal to those in the nicols. This is known as the diagonal 

 position. One of the directions of vibration in the section will 

 then be parallel to the slot. To ascertain which it is, the stage is 

 rotated through 45 till the direction which was parallel to the 

 slot is in the right and left position, when the index reading will 

 be that of the direction required. The same reading may be 

 obtained by adding or subtracting, as the case may be, 45 to or 

 from the index reading in the diagonal position. For instance, 

 if the slot is in the position shown in fig. 3, 45 will be 

 added. 



In the diagonal position the vibrations which pass the lower 

 nicol are resolved along the two directions of vibration in the 

 section. If there were no relative retardation between the 

 vibrations in these directions, thev would on emergence re- 

 combine to form, once more, vibrations parallel to the direction of 

 the vibration of the light when it left the lower nicol and would 

 therefore be extinguished by the upper nicol. As a result, how- 

 ever, of the relative retardation this is no longer the case, and 

 the various colours of the spectrum are transmitted in different 

 degrees, so that the compoimd tints known as interference colours 

 are obtained. These are dependent on the amount of the relative 



