Abstract
The demonstration conoscope described is a teaching aid in optical mineralogy that allows group observation of interference phenomena. It is essentially a petrographic microscope with its tube removed and replaced by one half of a ping-pong ball and a sheet of polaroid. The latter acts as the analyzer of the microscope, while the former acts as a projection screen for the interference figure. Simple modifications—e.g., using a liquid-filled hemisphere, sketching light vibration directions on its surface, and calibrating the hemisphere to measure 2V—greatly increase the utility of the instrument. Special thin sections can be prepared to show the effects of change in mineral orientation, thickness, and 2V. The quartz wedge and the accessory plates can be inserted and even rotated beneath the suspended hemisphere. These and other uses emphasize the conoscope's instructional value and suggest that it has possibilities as a determinative instrument.
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