L98 F. E. Wright — Transmission of Light through 



rotatory effects of the surfaces of the condensor lenses, which, 

 for the moment, may he disregarded. That this is the case, is 

 tacitly assumed in all microscopic work since the rotatory 

 effects produced by the condensor and objective lens systems 

 alone, on the plane of polarization of transmitted light •waves, 

 are practically negligible and the field appears approximately 

 dark under crossed nicols. Thus, in fig. 9, if the direction 7J 

 be the axis of the optical system of the microscope, Y'Z' the 



Fig. 9. 



plane of vibration of the entering waves, and P the direction 

 of propagation of one of these waves after refraction, its direc- 

 tion of vibration will then be along T, at right angles to P and 

 in the original plane of vibration. This same direction of vibra- 

 tion, OT, obtains for any other point, P', in the polar plane to 

 T. A wave propagated along P', but still vibrating along OT 

 in the original plane of vibration, will be destroyed by the total 

 reflexion in the analyzer, just as is the wave OP. Since the 

 entire field may be covered with waves similar to OP', whose 

 directions of vibration are contained in the plane of vibration 

 Y'Z', all the waves of the converging cone from the condensor 

 and objective systems are extinguished by the analyzer and the 

 field appears dark between crossed nicols, provided no bire- 

 fracting crystal plate intervenes. The effect of the lens system 

 of the microscope is, therefore, to change the directions of pro- 

 pagation of transmitted light waves, but not seriously to affect 

 the plane in which their vibrations take place. 1 Conversely, 



1 That there is some effect on the planes of polarization of transmitted 

 light waves is at once evident, even without accurate measurements, from 

 the lack of uniformity in illumination of the field when viewed under crossed 

 nicols in convergent polarized light. A dark cross divides the field into qiiad- 

 rants which are perceptibly lighter than the bars of the cross. This cross is 

 visible in every microscope and is not always due to faulty construction of 

 the objectives nor to strains in the glass. 



