302 WRIGHT: INTERFERENCE FIGURES IN OBJECTIVES 



incident wave is not rotated on transmission {B =0); (2) for a 

 given angle of incidence i the maximum rotation is obtained when 

 the wave front normal is included in the diagonal plane between 

 the principal planes of the crossed nicols (azimuth angle A = 45°) ; 

 (3) the angle of rotation increases with the angle of incidence i. 



Now a spherical surface may be considered to consist of a 

 series of minute planes inclined at all angles with the vertical 

 and in all azimuths. The rotatory effect of such a surface on 

 transmitted plane-polarized light waves is, therefore, different in 

 different directions, the result being a distinct uniaxial cross 

 with quadrants whose intensity of illumination increases with the 

 distance from the center. This is, in brief, the explanation of the 

 faint uniaxial cross which appears in all high power objectives 

 between cross nicols. The plane polarized light waves whose 

 normals are parallel to the principal planes of the nicols suffer no 

 rotation, while all others are rotated to an increasing extent as 

 their azimuth A increases until the maximum rotation at 45° is 

 reached. The reasons why these phenomena are so much more 

 distinct in high power than in low power objectives are, (1) the 

 larger numerical aperture of high power objectives and (2) the 

 fact that in such objectives the front lens of the system is a small 

 uncorrected glass hemisphere at whose steeply inclined sides 

 the transmitted light waves are rotated through relatively large 

 angles. 



If now a sensitive tint plate be inserted along the NW-SE 

 diagonal, between crossed nicols either below the condenser or 

 above the objective the quadrants of the faint interference cross, 

 observed in the upper focal plane of the objective, appear 

 differently colored, the northeast and southwest quadrants 

 being blue-green and the remaining two quadrants orange 

 yellow, or vice versa if the sensitive tint plate be inserted along 

 the NE - SW diagonal. The arms of the interference cross assume 

 the color of the sensitive tint plate irrespective of the direction 

 along which it is inserted. These interference colors appear 

 much more distinct when the sensitive tint plate is inserted so 

 that its vibration directions include only a small angle (1° to 10° 

 depending on the intensity of illumination) with the principal 

 nicol planes. 



