S6 THE POPULAR SCIENCE MONTHLY. 



strument 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 tele- 

 scope, 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 assist- 

 ance, 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 Coperni- 

 cus. 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 



