Since the publication of the Third Edition of this work, the author has been taken from us, and it has fallen to my lot to revise it and bring it up to the present time. This work I have approached with the greatest diffidence, having to follow one who had such profound knowledge of the subject, and I have earnestly endeavoured, as closely as possible, to act as I think he would have done had he been alive, and having enjoyed over twenty years of the happiest and closest business relations with him—actively co-operating in bringing many of the instruments to their present state, I venture to hope that I have to some extent carried out what his wishes would have been.

I have carefully read over and corrected the whole work, and the additions to it are only in the nature of bringing it up to date.

H. T. Tallack.

286, High Holborn,

June, 1914.