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apprehended until it has been invested in the apposite mental image. The illustrations and experiments in this book, and the diffraction grating which is supplied in the pocket of the cover, are one and all provided to enable the reader to attain to an adequate apprehension of the statements in the text.

Lastly, if the reader feels that he has a grievance in the fact that a number of new coined terms are employed in this treatise, I would put it to him that a complete vocabulary of technical terms is as essential to a satisfactory exposition as a proper ordering of the subject matter, and the provision of illustrative experiments.

It is in obedience to this consideration that I have, wherever the vocabulary of optics seemed to me to be deficient, coined, or adopted into currency, such new technical terms as seemed to me to be required.

To the objector who urges that the existing vocabulary already adequately subserves the exchange of ideas among those who are conversant with optics, and that the coinage of new words could quite well be avoided by occasional circumlocution and by resort to mathematical formulae, I would submit the following for consideration.

We have here, not the case of an interchange of ideas between physicists conversant with their subject and capable of threading their way through the pitfalls provided by an inadequate and often ambiguous vocabulary, but the case of an author endeavouring to give a clear exposition to a reader who must be assumed to be approaching the subject de novo.

Circumlocution and mathematical formulae might, no doubt, be resorted to, but circumlocution is everywhere a temporary, and at all times a difficult, expedient; and the use of mathematical signs as a substitute for speech can be defended only in the case of the inarticulate classes of the learned.

In bringing my task—a task which has occupied many years—to a conclusion, I desire to make it clear that I owe to my friend, Mr. J. W. Gordon, a debt greater than I can express for manifold intellectual help and suggestion. I am rather understating than