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materials employed are of the cheapest and most common description, and all the experiments may be performed at an expense of less than fifteen dollars. The apparatus is, at the same time, suitable for regular daily use in both the home and school, and with care should last for years.

The origin of this series of books, and the manner of their production, may be briefly stated. For several years Professor Mayer has been studying how to give to every teacher and scholar the knowledge of the art of experimenting. To accomplish this very desirable object, he had invented the simplest and cheapest apparatus, and he and his scientific friends had been satisfied with their performance. It remained to describe these instruments and the ways of using them. He found, however, that his leisure from professional duties was not sufficient for this work, and, not to delay further the publication of his labors, Professor Mayer called in Mr. Charles Barnard to assist him in preparing the books for the press. The construction and arrangement of the instruments were explained to Mr. Barnard, and the experiments were made before him. Mr. Barnard then wrote out the descriptions, which were revised by Professor Mayer. The engravings have been made under Professor Mayer's special direction, and care has been taken to render them accurate representations of the apparatus and experiments. PAGE

The nature of light is not touched upon in this volume. The authors propose to explain, in another book, the phenomena of interference and polarization of light, and to explain fully the structure of the eye and the nature of vision.

THE AUTHORS.

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