The works of Thomas Dick . ntricate dia-grams by which its doctrines have been illus-trated, yet it requires only the knowledge of a fewsimple facts and principles to guide an intclli»gent mechanic in his experiments, and in theconstruction of its instruments. In order to theconstruction of a refracting telescope, it is onlyrequisite to know, that the rays of light, passingthrough a convex-glass, an image of anyobject directly before it, at a certain point be-hind H, called iHs focus; and that^^^is imagemay be viewed and magnified by another convex-glass, placed at a certain distance be
The works of Thomas Dick . ntricate dia-grams by which its doctrines have been illus-trated, yet it requires only the knowledge of a fewsimple facts and principles to guide an intclli»gent mechanic in his experiments, and in theconstruction of its instruments. In order to theconstruction of a refracting telescope, it is onlyrequisite to know, that the rays of light, passingthrough a convex-glass, an image of anyobject directly before it, at a certain point be-hind H, called iHs focus; and that^^^is imagemay be viewed and magnified by another convex-glass, placed at a certain distance behind , let CD, fig. 1, represent a convex-glass,whose focal distance CE is 12 inches; let ABrepresent a distant object directly opposite; therays of light passing from this object, and cross-ing each other, will form an image of the objectAB, at EF, in an inverted position. Let GHrepresent another convex-glass, whose focaldistance is only one inch. If this glass is placedat one inch distant from the image EF, or 1$. APPLICATIONS OP OPTICAL KNOWLEDGE. Tl (hches from the glass CD, and the eyo applied atthe point S, the object AB will be seen turnedupside down, and magnified in the profortion of1 to 12, or twelve times in length and forms what is called an Aitnmomical tele-scope ; but, as every thing seen through it ap-pears inverted, it is not adapted for viewing ter-restrial objects. In order to fit it for viewingland objects, two other eye-glasses, of thesamefocal distance, (namely, one .inch,} are requi-site ; the second eye-glass IK is placed at 2inches from GH, or double their focal distance,and the glass NO at the same distance firom IK.*By this means a second image IM is formed inan upright position, wliich is viewed by the eyeat P, through the glass NO, and the object ap-pears magiUfii^ in thesame proportion as magni^vig power of a telescope of thisconstruction is found by dividing the focal dis-tance of the object-glass by the focal distan
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