540 WRIGHT: MEASUREMENTS OF REFRACTIVE INDICES 



is then derived from equation (8) and from it and 2q,2 a second 

 value for a obtained, which will probably be either higher or 

 lower than that originally assumed; a second value for a is ac- 

 cordingly chosen and the computation repeated; the new value 

 of a will probably also be too low or too high. If now the four 

 values thus obtained be plotted on ordinates to scale and a 

 straight line be drawn connecting the assumed values, and a 

 second line connecting the computed values, the intersection of 

 these two lines furnishes a value of a which is sufficiently exact 

 for most purposes. Analogous methods of procedure can be used 

 in solving for two unknown refractive indices, or for path- 

 differences, or for wave length of light employed. 



In the special case of a section normal to one of the bisectrices 

 the optic axial angle can be computed with a fair degree of 

 accuracy from measurements on interferences fringes, even 

 though the actual optic axes are outside the field of view. For 

 this purpose, several interference lines are measured to obviate 

 the determination of 'the thickness of the plate. In case only 

 one interference band is visible the method is still applicable, 

 provided a plate of known path-difference (e.g. | X) be inserted 

 below the condenser and the path-difference of the interfering 

 waves be increased or decreased a definite fraction of a wave 

 length. 



Measurements on crystal plates. 



(1) Uniaxial. Plate normal to axis. (Equation g) 



(a) Calcite plate (V. and H. collection No. 22). Optically negative; 

 d = 0.233 mm. (computed); X = 0.000589 mm.; co = 1.658; e = 1.486. 



fcl 2345678 



i 10?3 15?0 18?4 21?4 24?0 26?5 28?5 30?5 

 e 1.477 1.485 1.485 1.483 1.486 1.488 1.486 1.485 

 With the exception of the first value of e in this table (interference 

 band too wide for accurate readings) the computed values of e do not 

 vary greatly and the average value, e = 1.4854, is sufficiently accurate 

 for most purposes. 



(b) Zircon plate (V. and H collection No. 26) optically positive; 

 d = 0.545 mm. (computed); X = 0.000589 mm.; w = 1.930; e = 1.983. 



k I 2 3 



i 26? 1 38?8 48?0 



€ 1.984 1.984 



