THE MICROSCOPE BY MEANS OF THEIR OPTICAL CHARACTERS. 611 



means of the dark and light bands which replace the colours 

 in mono-chromatic light. The error may be thus determined 

 within ten micro-millimetres. 



Gypsum Plate. If the relative retardation be very small it is 

 difficult to detect or measure it by a quartz wedge on account 

 of the imperfection of the thin edge of the latter. It is best 

 investigated by means of a gypsum plate, parallel to the clino- 

 pinakoidal cleavage, of such a thickness as to show the violet 

 corresponding to a relative retardation of 575 micro-millimetres. 

 A very small decrease in the relative retardation is sufficient to 

 modify the colour considerably and cause it to pass into purple 

 or red, while a slight increase changes it to indigo or blue.* 



The gypsum plate is usually cut with its length parallel to 

 the fast direction.! It may be inserted in either slot or in any 

 other place in the course of the light between crossed nicols, but 

 always in a diagonal direction. If a crystal section with low 

 birefringence is now placed on the stage with its directions of 

 vibration parallel and at right angles respectively to this 

 direction, the colour of the plate will be seen to be modified 

 so as to indicate an increase or decrease in the relative retarda- 

 tion. In the former case the vibrations in the crystal parallel 

 to the slot will be fast, in the latter slow. J 



To determine the relative retardation of the crystal section, 

 that of the combination is determined by means of the quartz 

 wedge and the position of the black band on it. The stage is 

 then rotated through an angle of 90 and the determination 

 repeated. Half the sum will be the relative retardation of the 



* Gypsum plates are, however, usually made to show the red of the 

 first or second order, which is not so sensitive to variations in thickness 

 and therefore easier to produce of a practically uniform tint. 



f A circular plate mounted in wood is to be avoided, for if it becomes 

 loose, as frequently happens, it loses its correct orientation. The 

 plate should be marked so as to show the numerical amount of the 

 relative retardation and the character of the length, as in fig. 5. 



J In the case of small minerals with low relative retardation, which 

 are rendered inconspicuous by the bright light to which the gypsum 

 plate gives rise, it is better, if the construction of the microscope permits, 

 to insert the plate in a direction making only a small angle with the 

 cross wire. This diminishes the illumination due to the plate without 

 affecting appreciably the illumination and colour of the mineral under 

 examination (F. E. Wright, Am. Journ. Sc, series IV., vol. xxxv., 

 1913, p. 66). 



