AN ACCESSORY LENS FOR OBSERVING INTERFER- 
ENCE FIGURES OF SMALL MINERAL GRAINS 
ALBERT JOHANNSEN 
University of Chicago 
As long ago as 1880 Bertrand" discovered that in the bubbles 
which are found so frequently in the Canada balsam between the 
mineral section and the cover-glass, there may be seen an inter- 
ference figure of the mineral lying below it. The bubble acts as a 
lens of short focal length and takes the place of the objective, while 
the ocular and objective combined act as do the Bertrand lens and 
the ocular in the usual method of observing interference figures. 
Later, Schroeder van der Kolk? carried this method a step 
farther. He placed a drop of glycerine upon the cover-glass of the 
slide to be examined and stirred it rapidly with a thin rod so that it 
became filled with small bubbles. Over this he placed a cover- 
glass and, between crossed nicols and with a medium-power objec- 
tive, he depressed the tube a very little below the focal plane and 
found in each bubble a small interference figure. To avoid the 
necessity each time of preparing anew a glycerine foam, Schroeder 
van der Kolk used the following simple device: He placed a drop 
of Canada balsam on an object-glass and, if necessary, cooked it. 
After producing foam by rapid stirring, he placed a cover-glass 
upon it and the instrument was completed. To use it he placed it, 
cover-glass downward, upon the rock section and shoved it into 
such positions that bubbles appeared over the desired spots. 
In using this method, the present writer found that he could 
not produce bubbles in well-cooked balsam. If undercooked, the 
gum would squeeze out at the sides and stick to the rock slice under 
examination and the bubbles would become distorted from their 
tE. Bertrand, ‘“De V’application du microscope a l’étude de la minéralogie,”’ 
Bull. soc. min. France, II (1880), 93-96. 
2J. L. C. Schroeder van der Kolk, ‘‘Ueber eine Methode zur Beobachtung der 
optischen Interferenzerscheinungen im convergenten polarisirten Lichte, insbesondere 
in Gesteinsschliffen,” Zeztschr. f. wiss. Mikroskop., VIII (1891), 459-61. 
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