386 Wright — Two Microscopie-Petrographical Methods. 



tive index be higher than that of the liquid, it will act as a 

 lens on the transmitted light and tend to converge the light 

 toward the center (fig. 1) ; if lower than the surrounding liquid, 

 the effect will be reversed (fig. 2). By cutting off a part of 

 the rays (shaded parts of figs. 1 and 2) the light becomes one- 

 sided in its effect and then exhibits characteristic phenomena. 

 If the fragment AB (fig. 1) be observed under the microscope, 

 the edge AC of the mineral just j)rotruding from the darkened 

 space BC into the light will not be reached by so many beams 

 of light as the surrounding field, and hence will appear darker, 

 l 2 



Id -Q.g. 2, however, the reverse is the case ; the point A just 

 outside the dark field is lighted by all the beams between A 

 and C which are thrown together by the influence of the min- 

 erals ; A will, therefore, appear brighter than the surrounding 

 field. Moreover, as the dispersion is usually much greater for 

 fluids than for solids, it may happen that if the refractive index 

 for yellow rays is the same as that of the enclosing liquid, its 

 index of refraction for red may be lower and for blue rays 

 higher, in which case the mineral acts as a lens for the red rays 

 and converges them toward the center (fig. 1), while the blue 

 rays will be concentrated along the outer edge, A, which will 

 then appear blue. When, therefore, the refractive indices of 

 mineral and liquid are practically the same, bright colored 

 bands appear along the edge of the mineral. Mr. Schroeder van 

 der Kolk describes several experiments which show this phe- 

 nomenon in an elegant manner. He darkens half the field 

 by placing a small platinum plate over one half the condenser 

 lens beneath the stage. 



An adaption of this method, which can be effected with but 

 slight changes, has been found serviceable in petrographic 

 work. In the practical application of the method, the low 

 power is used, condenser lens slightly lowered, and half the 

 field darkened, not by means of the platinum plate mentioned 

 above, but by placing the finger in front of the reflector below 

 the stage and thus casting a shadow over any desired part of 

 the field. The same effect can be produced by moving the 



