Page:Popular Science Monthly Volume 24.djvu/96

86 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. 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. 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. 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. And for some purposes a poor instrument is as good as the best: an egg or a potato gives the housewife all the advantages, in measuring the density of her brine, which she would derive from the most skillfully-constructed hydrometer, or the most accurate balance and specific-gravity bottle. Galileo, with his simple-lens telescope, saw what, perhaps, never man before saw—viz., the moons of Jupiter; and by exhibiting the partial illumination of Venus, with the same imperfect instrument, he removed one of the strongest objections raised against the heliocentric theory of Copernicus. A word to the wise is enough. To my fellow-students I say: Whatever may be your several lines of study, get real knowledge, where possible, by seeing and handling things for yourselves; and, if you can not possess or have the use of a good instrument, do not therefore refuse the assistance of a poor one; but in all cases get and use the best you can. Rembrandt made pictures with a burned stick before ever he possessed pigment or pencil.

The lenses requisite for such a telescope as I have constructed, and shall describe, can be purchased of an optician by those who live in large cities; those who reside at a distance may have them sent by mail at a trifling additional cost. They are: 1. An achromatic object-glass, one and a half inch diameter, with a focus of thirty inches. 2. Two plano-convex lenses of the respective foci of two inches and three fourths of an inch. The object-glass will cost about two dollars, and the other two lenses about seventy-five cents each.

Now procure a straight cylindrical roller of pine, two and five eighths inches in diameter, and thirty inches long; procure also a roller seven eighths of an inch in diameter, and fifteen or sixteen inches long. These are for forming the tubes on. Take stout brown wrapping-paper, and, with book-binder's paste, form a tube, twenty-nine inches long, on the large roller. Spread the paste on evenly, and rub the several layers of paper down smoothly with a cloth. Nine or