MICROSCOPICAL PETROGRAPHY 495 
In this series the refractive index of each liquid for sodium 
light is 0.005 higher than that of the liquid immediately preceding 
it. In choosing these liquids the guiding factors were stability, 
miscibility, and low dispersion. The refractive indices were 
measured on a total refractometer up to 1.74; above this by the 
hollow-prism method in sodium light. In place of the hollow 
prism Dr. Merwin has recently found a prism satisfactory which 
is made by fusing two narrow strips of plane glass (selected micro- 
scope object glass) together at one end so that their plane surfaces 
below the joint include an angle of 35° to 45°. A drop of the highly 
refractive liquid is then placed in the wedge-shaped space between 
the glass plates and adheres by capillarity to their plane surfaces, 
thus assuming the required prism shape. The liquids are kept 
conveniently in small dropping bottles with ground-glass stopper 
and cap, which interposes two ground joints to prevent evapora- 
tion. Experience has shown that the liquids so kept do not vary 
over 0.002 in a year, while the average change in the refractive 
index of a liquid is about o.oo1 decrease for every 3° C. rise in 
temperature. By using obliquely incident light or by observing 
the Becke line, it is possible to ascertain at a glance whether the 
refractive index of a particular grain is above, below, or about 
equal to, that of the liquid. 
In case the mineral grain has a higher refractive index than 
the liquid, it acts as a lens on an incident pencil of rays and increases 
their convergency; if its refractivity is lower than that of the 
liquid, it diverges the incident rays. This difference in behavior 
is best shown by a pencil of oblique rays; these are concentrated 
on the distant side of a higher refracting mineral grain opposite 
to that at which the incident rays impinge; in a lower refracting 
grain they are concentrated on the near side. In both instances 
the two margins of the mineral grains appear unequally illuminated; 
if the light be incident from the left, the bright band of light appears 
on the right margin of the higher refracting grain, and on the left 
margin of the lower refracting grain. In case both mineral and 
liquid have the same refractivity for yellow or green light, the 
difference in dispersion between liquid and mineral gives rise to 
characteristic phenomena. ‘The dispersion for liquids is in general 
