(182.) 
Space-pe- 
netrating- 
power of 
telescopes, 
183.) 
840 
the atmosphere, especially in these climates, impose 
a speedy limit to the increase of magnifying power, 
unless in exceptional circumstances. The same ob- 
jections do not apply to the increased aperture 
and illuminating power of teleseopes. This con- 
sideration induced Herschel to construct his forty- 
feet reflector, the funds being advanced by the king, 
on the recommendation of the Royal Society. The 
speculum had a useful surface precisely four feet in 
diameter, the thickness three and a half inches, the 
weight when cast above 2000 pounds, The second or 
plane mirror having been dispensed with, the image 
was thrown to one side of the object-end of the tube 
by means of a very slight inclination of the speculum, 
and was there observed directly by a common eye-piece 
or single lens, the observer being thus stationed with 
his back to the object he is viewing. This vast in- 
strument collected a cone of rays from a distant point 
between six and seven times larger than the twenty- 
feet instrument commonly employed by Sir W. Her- 
schel, of which the aperture was 18-8 inches. This 
telescope was not frequently used, partly from the 
rarity of sufficiently steady weather, and from the 
difficulty of preserving the figure of the mirror under 
changes of temperature. Its figure was also affected 
by flexure under its own enormous weight, and it 
has been found one of the greatest difficulties by those 
who have followed Herschel’s steps to avoid this 
source of error. The forty-feet speculum has been 
religiously preserved by the filial care of Sir John Her- 
schel, in whose possession the writer of these pages 
had the pleasure of seeing it, now many years since, 
in a state of high polish, It remains in a deposit 
hermetically closed, at Slough.? 
Herschel made an ingenious determination of what 
he called the space-penetrating-power of telescopes, by 
which he discriminated the relative distances at which 
a given fixed star would become invisible in his 
several instruments. It was grounded on the as- 
sumption that the visibility of the object depends, 
first, on the density of its light reaching the eye, and 
secondly, on the number of rays of that density 
concentrated in the image. The former quantity 
varies inversely as the square of the given distance, 
the latter directly as the square of the aperture of 
the telescope ; the limiting visible distance therefore 
of the radiant body will be simply as the aperture. 
To this he applied corrections depending on the loss 
of light by reflection at the specula, and by trans- 
mission through one or more eye-glasses. On this 
scale the space-penetrating-power of the seven-feet 
Newtonian being twenty, that of the twenty-feet tele- 
scope (front view) was seventy-five, and the forty-feet 
telescope 192, These estimations, as we shall see, 
form an important step in Herschel’s generalizations. 
Of new bodies belonging to our system he dis- 
ASTRONOMY.—SIR WILLIAM HERSCHEL. 
[Diss. VI. 
covered Uranus and his six satellites, the two satel- 
lites of Saturn next to the ring, and several comets, 
He did not, so far as we know, even suspect the 
planetary character of Uranus, which he believed to 
be a comet. The honour of this appears to be due 
partly to Saron of Paris, partly to Lexell of St 
Petersburg. Arago supposes that Laplace had 
also a share, but the evidence on the whole matter 
is somewhat obseure. Unquestionably it was an ex- 
ceedingly difficult matter to fix upon a correct orbit 
for a body moving with such extreme slowness within 
the limits of the first few weeks after its discovery, 
and its distance was far beyond what any one calcu- 
lating a comet’s path would readily assume. The 
original discovery was also a fortunate one, for but 
eleven days previously the planet was in a position 
apparently stationary. It has led to results of still 
higher interest by the prediction of the existence and 
place of Neptune from the irregularities of Uranus. 
The satellites move almost perpendicularly to the 
ecliptic. Strange to say, they remained for, I be- 
lieve, more than half a century, unseen by any eye but 
that of their discoverer and his son: and two of them 
still stand recorded on the single authority of Sir W. 
Herschel. They are the least specks of light which 
optical power has ever made visible, 
Herschel made many observations on the physical 
appearance of the sun, moon, and nearer planets, and 
the times of their rotation. Into these details we 
cannot now minutely enter. The papers which record 
them are characterized (like all those of the same 
author) by the faithful minuteness with which the ob- 
servations are detailed,and by thementionof every in- 
fluential cireumstance by which the results might be 
affected. In the case of obgervations so exceedingly 
delicate as those of the planets, the astronomer has to 
keep up an incessant struggle between the anxiety to 
record all that he sees, and that of recording nothing 
more than he really and perfectly sees. One or two 
glimpses are not sufficient. Herschel at one time 
imagined that the new planet had one or more rings 
which unquestionably do not exist. His elaborate in- 
vestigation of the figure, belts, and rings of Saturn, the 
incredible tenuity of the latter, and the fact of their ro- 
tation in 10" 32™, were amongst the most interesting 
of his observations, and were illustrated by drawings 
made with remarkable care. The edge of the ring was 
seen in 1789, when it had totally disappeared in all 
but his forty-feet telescope, and he estimated that its 
thickness could not exceed 100 miles. How astonish- 
ing the magnitude and thinness of this stupendous 
circular plane, or rather series of flat rings! Some 
idea of it may be deduced from the fact that if the 
thickness of the paper on which this is printed be 
taken to represent that of the rings, their greatest 
diameter would correspond to nineinches. Observa- 
1 The first object viewed by Sir W. Herschel with his great telescope in 1789 was the nebula in Orion; possibly it was also 
the Jast, for he again observed it in 1811 (See Phil. Zr. 1811, p. 322). 
2 See Weld’s History of the Royal Society, ii., 198, 
Herschel’s 
discovery 
of Uranus 
(or Georgi- 
um Sidus); 
(184.) 
observa- 
tions on the 
planets ; 
a 
