2. On a New Method of shewing the Unequal Expansion of Crystals

Extract

This method is founded on the expedient devised by Mitscherlich, of obtaining two images of a minute distant object from a section of a hemitrope crystal of selenite, which is flat when cold, but is composed of two inclined surfaces when heated. By throwing the sun's image, reflected from such a surface, on a distant wall, the gradual formation of the double image during heating may be seen by any number of persons.