630 TELESCOPE We are now able to substitute for the heavy and intractable speculum metal a disk of glass which is far easier to cast and anneal, and be- ing much firmer can be made of less than half the weight of the metallic mirrors. The helio- scope, for observing the sun, is a telescope with the aperture diminished as much as possible, and usually provided with shades of stained glass to protect the eye. Still, great inconve- nience is felt from the intense heating pow- er of the concentrated solar rays. Sir John Herschel proposed to use only the very small portion of light reflected from the first surface of glass, by constructing the large mirror of a Newtonian telescope of a double-concave, well polished lens, whose first surface only is tru- ly figured to serve .as reflector for the 2 -6 per cent, of rays untransmitted and unabsorbed. The lower end of the telescope tube being left open, all the remainder of the light passes out and is dispersed. But even the small amount of reflected rays is still further reduced by the second reflection, which is made to take place at the first surface of a prism whose refracting angle should not be less than 30 or 40, so that now the portion of light finally reaching the observer is but T7 Vg- of the direct illumina- tion, in consequence of which immense reduc- tion a very light shade only is needed. Porro of Paris, in constructing a telescope upon this principle, improved it by placing the prism for the second reflection at the polarizing angle for glass, whereby, upon introducing a Nicol's prism, the light may be enfeebled as much as de- sired without using any shade at all. The great requisites of a telescope stand are firmness and stability, combined with a facility of motion which will allow the instrument to be pointed with ease and certainty to any part of the heavens. Fraunhofer, whose plan is now gen- erally followed, adopted the equatorial form, as it is called, which consists essentially of a polar axis upon which the whole instrument is moved parallel with the celestial equator, and which carries in a socket another axis at right angles to itself, upon which latter the telescope moves from or toward the" pole. By the combined motions command of the whole visible hemisphere is given, and with the ad- vantage that, the instrument being once di- rected to a star, the observer can follow it in its diurnal path by motion upon the polar axis alone ; moreover, by application of a simple train of wheelwork this motion can be effect- ed by machinery, and the observer is thus en- abled at his leisure to contemplate or to mea- sure the objects which appear fixed as though in an immovable sky. In the immense English reflectors, the lower end of the tube rests upon the ground or some solid support, and even then for the needful motions of the instrument powerful appliances of machinery have been required ; but in latter days mechanical engi- neers have been able so to combine and coun- terpoise great masses of cast-iron machinery as to effect with wonderful ease every delicate movement desired by the astronomer, and now the idea of mounting even these large tele- scopes equatorially is growing familiar. The application of clockwork movement to such large reflectors renders it practicable to use them for celestial photography, as well as for some extremely delicate astronomical mea- surements. The application of the telescope to meridian instruments will be exemplified in the article TRANSIT CIRCLE ; but the telescope is also universally used for differential mea- surements. For such observations various modifications or appliances have been from time to time suggested or practised. The filar micrometer is the most common auxilia- ry of the telescope, and in skilful hands is ca- pable of astonishing accuracy. (See MICROM- ETER.) Great use has also been made of the power of producing and comparing together double images of the objects to be measured. These double images are produced in various ways. Savery in England in 1743, and Bou- guer in France four years later, proposed, in- dependently of each other, to measure the di- ameter of the sun by using two object glasses in the same telescope and with the same eye piece. In Savery 's plan the glasses were all fixed so as to give two images of the sun whose outer edges were nearly in contact ; and by measuring the variable distance of these edges, he obtained the corresponding variations of the semi-diameter from perigee to apogee. Bouguer made one of his object glasses movable, and thus could measure any angle from zero to his maximum limit, which was probably somewhat greater than the sun's diameter. In 1753 John Dollond invented the divided-object-glass micrometer, which has in later years, under the technical name of helb- ometer, achieved such wonders in the hands of Bessel and his followers. In this instru- ment the object glass itself when finished is divided into two equal segments, each of which forms its image independently of the other. "When the semi-lenses are brought to their normal position of coincidence, the two im- ages coincide also; but when separated, the images diverge, and the angle of divergence is measured by the amount of separation of the lenses. Thus the apparent diameter of a planet, for instance, is obtained by separating the images until their outer edges are in exact contact, and this may be more accurately per- ceived than the coincidence of the edge with a fine thread placed tangent to it as in the filar micrometer. Dollond proposed moreover to gain both accuracy and convenience of use by placing a divided object glass of very long focus before the speculum of a reflecting tele- scope, which would give a larger scale for the measurement of a given angle than would be- long to a simple telescope of the fame length. Fraunhofer was at the time of his death en- gaged in devising a heliometer which, when afterward completed, was placed at Kuni^s- berg. Bessel, whose " Theory of the Ileliom-
