PHYSICAL ASTRONOMY. 159 
The focus consequently was formed at the anterior edge of the instrument, 
where it was received by the ocular. The observer sat directly in front of 
the open tube looking through the ocular. 
In this manner Herschel constructed his 20 and 30 foot telescopes ; 
the 20 foot with a speculum of 10 inches diameter. A seven foot reflector 
finished by Herschel in 1780 was of great excellence ; with it he discovered 
Uranus on the 18th of March, 1781. The magnifying powers were 230, 
460, and 930; but to his greater reflector Herschel could apply powers of 
500-2000, without overtasking them for strongly illuminated objects. The 
giant 40 foot telescope completed in 1789, is represented in pl. 15, fig. 1. 
The tube, DD, constructed of sheet-iron, was 40 feet long, 4 feet 10 inches 
in diameter, and the whole telescope weighed about 5100 pounds; the great 
speculum alone weighed 2148 pounds. The magnifying powers of the 
instrument were, for the planets, 250 and 500; for the fixed stars, 1000- 
6400. The distinctness of the objects seen is said to have been astonish- 
ingly great. The cost of the whole apparatus amounted to about £2000 
sterling. During observations with this colossal. instrument, Herschel sat 
at the side of the tube at its upper end, in a frame H, fixed to the ladders, 
G, G, which accompanied the tube in its movements. He thus looked into 
the instrument, with his back to the star, and examined this latter directly 
with the eye-glass. Unfortunately the mirror, during a single damp night, 
lost its polish, and the whole instrument in a few years after its construc. 
tion was entirely useless. The figure gives, without the necessity of further 
explanation, an idea of the strong scaffolding between which the telescope 
could be moved in a perpendicular direction by means of several ropes, 
AE, FE; the horizontal motion of the whole apparatus, scaffolding and 
tube, was produced by a rotation by means of rollers running upon the 
periphery of a horizontal circular railway, ABAB. Around and above the 
whole was built a round tower with a revolving roof, whose opening could 
be brought towards the part of the heavens to be observed. More 
recently Lord Rosse has constructed a gigantic telescope, more than 12 feet 
longer than that of Herschel, having a speculum of 6 feet in diameter. 
The Mural Quadrant. 
83. As long as astronomical observations did not possess that degree of 
accuracy now exhibited, the mural quadrant was, of all instruments used in 
measuring altitudes, the most useful. Pl. 15, fig. 19, represents the cele- 
brated mural quadrant of Tycho. This had a radius, DC, of eight feet, by 
means of which, aided by a vernier, EK, very small arcs could be read off on 
the limb CC. The iron grating, DCC, which formed the body of the 
quadrant, was fastened to a wall, GAA, placed in the meridian. and the 
rule DD, with the telescope, moved up and down on this grating. In this 
manner it was possible to observe not only the passage of the meridian by a 
star, but also its altitude or zenith distance. The depending plumb-line, 
DA, served to fix the quadrant in its proper position in the vertical plane. 
159 
