LENS FOR OBSERVING INTERFERENCE FIGURES 97 
spherical form. Another objection is that the extra cover-glass 
increases the distance between the balsam film and the mineral and, 
as a consequence, a dark border surrounds each bubble and cuts 
down, materially, the size of the interference figure. If a piece of 
glass, full of minute bubbles and of the thickness of a cover-glass, 
could be prepared, it would be a great improvement. By removing 
the cover-glass of the rock section to be examined and pressing the 
new glass down over it on a drop of oil, the interference figures 
would appear sharp, of the full size of the bubbles, and without a 
dark border. Having no such glass the writer has prepared small 
lenses as follows: 
A glass rod was heated over a Bunsen burner and was drawn out 
as thin as possible. Pieces of the glass threads thus obtained were 
again heated and again drawn out to hair-like thinness. These 
were broken into lengths of an inch and a half and the extremities 
held an instant in the edge of the flame, whereby truly spherical 
globules were produced at each end. After preparing a number of 
these spherical lenses, they were examined under the microscope 
and all that were not perfect or which contained bubbles were re- 
jected. Likewise only those which had a diameter of less than +} of 
an inch (0.2 mm.) were retained. If, now, such a lens is placed 
directly in contact with the cover-glass over the mineral to be 
examined, and the microscope arranged with crossed nicols, ocular, 
and a medium- or low-power objective (No. o to 4, Fuess), there 
will appear in it a small but perfect interference figure. The 
microscope should be focused upon the glass sphere and the tube 
then slightly raised. A condensing lens is not necessary but 
without it part of the figure is cut off by the dark border. The 
optical character and dispersion can be determined as well by this 
method as by the use of a Bertrand lens, and the figure is decidedly 
sharper. By its means it is possible to examine the interference 
figures, undisturbed by surrounding minerals, of grains smaller 
than is possible by the Lasaulx, Klein, or Bertrand methods, and 
it possesses the further advantage that the mineral and the inter- 
ference figure can be seen at the same time. By shifting the lens, 
the optical orientation of all of the grains in a section can be deter- 
mined. When used in connection with the Fedorow universal 
