Transcriber’s Note:
The cover image was created by the transcriber and is placed in the public domain.
Fig. 90.—Screen Picture in Polarised Light, with Nicols crossed, of a thick Plate perpendicular to the Axis of a naturally twinned Crystal of Quartz, the left half being of right-handed Quartz and the right half of alternately left and right-handed Quartz, the Planes of Demarcation being oblique to the Plate.
Fig. 97.—Crystals of Benzoic Acid in the Act of Growth, as seen on the Screen in Polarised Light with crossed Nicols.
CRYSTALS
PREFACE
The idea underlying this book has been to presentthe phenomena of crystallography to the generalreading public in a manner which can be comprehendedby all. In the main the sequence is thatof the author’s evening discourse to the BritishAssociation at their meeting at Winnipeg in thesummer of 1909. It is hoped, however, that thebook combines the advantages of sufficient amplificationof the story there told to make it anadequately detailed account of the developmentof the subject, and of the immense progress whichhas been made in it during recent years, with a fulldescription of the numerous experimental illustrationsgiven in the lecture, involving some of themost beautiful phenomena displayed by crystalsin polarised light. Such an account has not beenotherwise published, the brief abstract appearingin the Report of the British Association for 1909giving no account of the experiments, which werea feature of the lecture, owing to the employmentof a fine projection polariscope of more or less novelconstruction, and including two magnificent largeNicol prisms, a pair of the original ones made byAhrens. The author has been frequently requestedto publish a fuller account of this discourse, and asthe general plan of it so fully embodies the presentaspect of this fascinating science, it was determined,viwhen in