WRIGHT: A NEW HALF SHADE APPARATUS 309 



character of the interference figure when tested in the usual man- 

 ner by the insertion of a sensitive tint plate either above the 

 objective or below the condenser. 



« 



PHYSICS. — A new half shade apparatus with variable sensibility. 

 Fred. Eugene Wright, Geophysical Laboratory. 



The importance of variable sensibility in half shade appara- 

 tus to meet best the different conditions of illumination and obser- 

 vation has been frequentl}^ emphasized in recent years and sev- 

 eral instruments have been constructed which serve the purpose 

 more or less satisfactorily. The device described in the following 

 paragraphs is a simple, inexpensive half shade apparatus which 

 accompUshes its object well and merits a brief word of descrip- 

 tion. It is essentially the instrument used in experiment 5 of the 

 preceding article in which the theory underlying its construction 

 is outlined briefly. The essential elements of this theory are: 

 (1) a plane-parallel glass plate of refractive index n (tilted so that 

 it makes an angle i with the line of propagation of an incident 

 plane-polarized light wave, and also so that the normal to the 

 plane of incidence includes an azimuth angle A with the line of 

 vibration of the light wave), rotates the plane of vibration of the 

 transmitted light wave through an angle B which can be computed 

 from the Fresnel equation, cot B = cos- (i-r) . cot A; both experi- 

 ment and theory prove that the angle of rotation increases with 

 the refractive index ?i of the substance and with the angle of 

 incidence i; also with the azimuth angle A up to 45°; when A = 

 0°, 90°, 180°, the rotation is nil. For a given angle i the rotation 

 angle is a maximum for A = 45°. If, therefore, a plane-parallel 

 glass plate be mounted so that it can be rotated about a hori- 

 zontal axis in the first (NE) quadrant midway between the 

 principal nicol planes, the azimuth angle A for incident waves 

 from the polarizer is 45° for all angles of incidence i and the 

 angle of rotation of the transmitted waves can be calculated from 

 the simpUfied Fresnel equation, cot B = cos^ (i-r). If now a 

 second glass plate be taken and rotated about an axis in the 

 second (iVTF) quadrant, the azimuth angle of the incident light 

 waves from the polarizer is -45° and the Fresnel equation re- 



