"392 Wright: determination of refringence 



the low power objective (E. F. 16 mm.) and finally to reach the eye 

 of the observer. If a mineral grain immersed in a liquid of slightly 

 different refractive index be examined mider these conditions of 

 illumination, its edges appear in part brighter, and in part darker 

 than the field. The intensity of illumination of the field is so 

 weak that the illumination of the edges is clearly marked even for 

 differences in refractive index of only ±0.001, and the eye suffers 

 no appreciable strain in making the observation. If now the 

 upper blade be moved away from the edge of the image, a small 

 amount of direct light from the condenser enters the field, and the 

 phenomena produced by oblique illumination from the lower stop 

 are observed under reduced field illumination. As the upper 

 blade recedes, the field illumination increases until finally the 

 conditions of ordinary oblique illumination are reached. The 

 phenomena observed under the first set of conditions are, more- 

 over, the reverse of those produced on withdrawing the upper stop ; 

 the edges which appeared bright in the first case are dark in the 

 second, and vice versa. This reversal, caused by the shift of the 

 upper stop, is an additional factor which adds to the sensitiveness 

 of the method. The movable upper stop not only increases the 

 distinctness of the ordinary phenomena of oblique illumination by 

 reducing the field illumination, but it also enables the observer to 

 reverse the phenomena and to study the slight differences in 

 illumination against a dark field for which the eye is more sensi- 

 tive. 



It is of interest to note that the principle on which the first 

 part of the new method is based is that first used by Foucault for 

 testing the chromatic and spherical aberrations of a telescope lens. 

 The method was later used by Topler for detecting small differ- 

 ences in the refractive index of a medium, especially inhomogeneity 

 in optical glass. Topler's method is still used for this purpose, and 

 for testing the homogeneity of solutions especially with respect 

 to concentration currents; it has also been employed by R.W. 

 Wood to obtain direct photographs of sound waves. 



