The Berthon Dynamometer

Abstract

ABSENCE from home, and many engagements, have prevented an earlier reply to “W. R.'s” letter in NATURE, October 5. In my previous communication I believe I gave the address of the inventor, to whom I thought reference might naturally be made; in order, however, to meet “W.R.'s” wish, I will explain the construction of the very simple but efficient instrument in question. It is merely a V gauge, formed of two pieces of thin brass converging at a very acute angle, and graduated along one of the edges; the divisions being viewed through a lens held in the hand simultaneously with the image of the object-glass or speculum formed by the eye-piece; the diameter of that image is given at once by the divisions to 1/500 of an inch, and can be readily estimated to half that value. The arrangement mentioned by “W. R.” is no doubt very convenient, and quite adequate for his purpose; but for high powers I should suppose that the comparative coarseness of the engraved lines would make itself much more felt than it is in Mr. Berthon's invention, and the balance of economy is so greatly in favour of the latter in comparison with every contrivance with which I am acquainted, that I have no hesitation in saying that it ought- to be in the hands of every amateur who cares to know the magnifying power of his telescope. It may be procured for five shillings, of Mr. Tuck, watchmaker, Romsey.