304 



WRIGHT: INTERFERENCE FIGURES IN OBJECTIVES 



tensity of illumination is not uniform, the circular field being divided 

 into four quadrants, the dark dividing lines corresponding to the 

 zero isogyres of the interference figure. On insertion of the sensitive 

 tint plate these quadrants become brightly colored, two opposite quad- 

 rants in blue green tones and the other two in orange yellow tones. 

 The colors are remarkably brilliant and correspond in their distribu- 

 tion to that of a uniaxial optically negative mineral. If monochro- 

 matic light be used the angular rotation for each part of the field 

 can be determined by rotating the upper nicol and testing the position 

 of extinction by means of the bi-quartz-wedge plate and then, by use of 

 Fresnel's equation for the rotation of plane-polarized light waves on 

 reflection, the refractive index of the mercury can be computed for the 

 particular wave length of light employed. 



Experiment 5. Cement to the two sides of a small 45° total re- 

 flecting prism two strips of plane-parallel glass (carefully selected 

 cover glass strips serve the purpose well) so that the ends of the glass 



strips extend 6 or 8 mm. beyond 

 the edge of the prism. Drill a hole 

 transversely through a brass c^din- 

 der and insert a brass rod in the 

 same. Lacquer the inside of the 

 brass cjdinder with a matt black. 

 Cement the hypothenuse face of 

 the prism to this rod, so that the 

 prism edge is at right angles to the 

 axis of rotation of the rod (fig. 1). 

 If now the axis of rotation be 

 placed parallel with one of the 

 Fig. 1 principal nicol planes, the two glass 



plates can be rotated and in every 

 position the angle of incidence i for both plates is the same while the 

 azimuth angles A are of opposite sign. 



The effect of this arrangement is analogous to that of a half shade 

 apparatus, namely, that, if in the one quadrant the angle of rota- 

 tion of the transmitted plane-polarized waves is +A°, it is — A° in the 

 adjacent quadrant. If now the sensitive tint plate be placed be- 

 tween the crossed nicols, the color phenomena described in the pre- 

 ceding experiments appear clearly marked; by rotating the prism all 

 gradations between the undisturbed sensitive tint purple to bright 

 green in the first quadrant and yellow in the second can be obtained. 

 This experiment illustrates clearly the principle underlying the inter- 

 ference phenomena observed in the lenses of a high power objective. 



Experiment 6. Since the amount of rotation of refracted or re- 

 flected light waves at inclined surfaces of an anisotropic substance 

 depends on the difference in refractive index between the substance 

 and the surromiding medium, it is proper to infer that, in case a glass 

 bead be immersed in a liquid of the same refractive index, no rotation 

 will occur and hence no color phenomena will be observed. This is 

 approximately correct, though in many instances the color phenomena 



