But the use of a simple lens of two and a half inches focus annihilates three fourths of this distance, quadruples the angle of vision, and enables us to see objects only one sixteenth as large as the least we can see with the naked eye. Moreover, reflect!-so far as vision is concerned, men, on an average, without instrumental assistance, are inexorably kept at a distance from "things" of ten inches, and must view them under the angle thence subtended. We have but to remember the great strides made in the acquisition of knowledge by the aid of the very imperfect first-forms of every instrument which has been invented, to be assured of this. Some people conclude that, if they can not possess a first-class instrument of this or that kind, they are better off without any but a moment's consideration will show the fallacy of this conclusion, and that, on the contrary, even a very poor instrument of observation or precision, or generally of research, in aid of the senses-be it telescope, microscope, spectroscope, balance, thermometer, chronometer, or chemical reagent-is vastly better than none. I now lay before my fellow-readers the following directions which, step by step, I myself have put in practice, in making a really serviceable achromatic telescope, which will exhibit the moon's surface magnificently, and show very satisfactorily the spots on the sun's disk, the satellites of Jupiter, and other celestial phenomena. Indeed, it is not to be doubted that the popularizing of instrumental aids for experimentally verifying the teachings of scientific discoverers will form a notable part of the work of the future schoolmaster.Ī few years ago I derived great pleasure from successfully constructing a home-made microscope, guided by directions contained in "The Popular Science Monthly," at a time when my means did not enable me to purchase a good instrument from the optician. TO render easier of attainment instruments which assist in the investigation or contemplation of natural phenomena, and which supplement man's sense-organs, is to forward by so much the diffusion of real knowledge, and to aid the work of human enlightenment and progress.
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