Part 9 (1/2)

Theapparatus are on the pre tools and polishers are placed underneath the tube, let into the ground, and level with the surface of the gravelled area in which it stands”

The closing of the tube was done with appropriate cereh it by the fah, a poem, written by Sir JOHN, was read, the machinery put into its present position, and the tube sealed

The hout that HERSCHEL'S pri of the telescope itself, but that his mind was constantly directed towards the uses to which it was to be put--towards the questions which he wished it to answer

Again and again, in his various papers, he returns to the question of the _limit of vision_ As BESSEL has said:

”The naked eye has its liht of fainter stars than these does not affect the retina enough for them to be seen A very seneral, without doubt, to more distant stars

A more powerful one penetrates deeper into space, and as its power is increased, so the boundaries of the visible universe are widened, and the number of stars increased to millions and millions Whoever has followed the history of the series of HERSCHEL'S telescopes will have observed this But HERSCHEL was not content with the bare fact, but strove ever to kno far_ a telescope of a certain construction and size could penetrate, coations were never for the discovery of new facts concerning the working of his instrue of the distribution of the fixed stars in space itself that he stroveHERSCHEL'S instruned to aid vision to the last extent They were only secondarily for the taking of e of the _motions_, but of the _constitution_ and _construction_ of the heavenly bodies”

Besides the stands for his telescopes, which were both ingenious and convenient, HERSCHEL devisedthe art of observation His les, his la apertures, and thethese

Points in practical astronoh the years of observation A reference to his original papers will sho numerous, how varied, and how valuable these are I cannot forbear quoting here the account of a precaution observed during his examination of the belts on _Saturn_ (1794)

It is theexample of how fully HERSCHEL realized that the eye of the observer is a material part of the optical apparatus of astronomy Simple as this principle may appear, it was an absolute novelty in his day

Inthese observations, he says:

”I took care to bend my head so as to receive the picture of the belt in the same direction as I did formerly This was a precaution that occurred to me, as there was a possibility that the vertical diaht be more or less sensitive than the horizontal one”

Astronoestion of the processes which have led to important results in the hands of Dr OTTO STRUVE and others in the comparison of the measures of double stars by different observers, each of whom has a personal habit of observation, which, if not corrected,to avoid

_Researches on the Relative Brightness of the Stars: Variable Stars_

No research of HERSCHEL'S was more laborious than the elaborate classification of the stars according to their co the years 1796 to 1799 It was directly in the line of his main work--to find out the construction of the heavens

His first paper had been upon the variable star _Mira Ceti_ Here was a sun, shi+ning by its native brightness, which waxed and waned like theperiod invisible to the unassisted eye Then it can just be seen, and increases in brightness for a little over a month, and attains a maximum brilliancy

From this it decreases for nearly threeinvisible, remains so for five or six months Its whole period is about 333 days Are all other stars constant in brightness? The example of _Mira Ceti_ and of other known variables makes this at least doubtful

But the sun itself may vary for all that we know It is a simple star like the rest

This question of variability in general is an i accurate catalogues of the relative brilliance of stars at various tiue existed before HERSCHEL'S time, and led by the discrepancies in isolated cases, which he found between his own estimates and those of his predecessors, he ues, in which were set down the order of sequence of the stars of each constellation

The method adopted by HERSCHEL was perfectly sih most laborious in practice Suppose any nuh to each other to be well co down the names of the stars, A, B, C, etc, in the order of their relative brightness Thus if for a group of eight stars we have found at one epoch A, B, C, D, E, F, G, H, and if at another time the order was A, B, C, D, F, E, G, H, symptoms of variability are pointed out Repeated observations, where the same star is found in different sequences, will decide the question Thus, for the stars visible to the naked eye, we know exactly the state of the sky in HERSCHEL'S day, now nearly a century ago Any ues have been singularly overlooked by the observers of our generation who have followed this branch of observation, and it was not till 1876 that they received proper attention and a suitable reduction (at the hands of Mr C S PIERCE)

We owe to HERSCHEL the first trustworthy account of the stars visible to the naked eye, and since the date of his labors (about 1800) we have similar views published by ARGELANDER (1839), HEIS (1848), ARGELANDER and SCHoNFELD (1857), GOULD (1860 and 1872), and HOUZEAU (1875) Thus his labors have been well followed up

In the prosecution of this work HERSCHEL found stars whose light was progressively diularly increased, one star whose light periodically varies (_[alpha] Herculis_), and at least one star (55 _Herculis_) which has utterly disappeared On October 10, 1781, and April 11, 1782, he observed this latter star, but in May, 1791, it had totally vanished There was no trace re

The discovery of the variability of _[alpha] Herculis_ was a ht appear Up to that time the only variable stars knoere seven in number Their periods were four hundred and ninety-four, four hundred and four, three hundred and thirty-four, seven, six, five, and three days These periods seeroups, one of from three hundred to five hundred days, the other comparatively much shorter, of three to seven days

_[alpha] Herculis_ caroups, its period being about sixty days

The cause of these strange and regular variations of brightness was supposed by HERSCHEL to be the rotation of the star bodily on an axis, by which revolution different parts of its surface, of different brilliancy, were successively and periodically presented to us This explanation it ht have been difficult to receive, when the periods of the known variables were so e over the interval, and quite confirmed him in his belief He returned to the subject of the revolution of stars about their axes again and again, and connected it with the revolution of satellites

He found that the satellites of _Jupiter_ and one of _Saturn's_ periodically changed in brightness, and by quite simple means showed that their periods of rotation were at least approximately the same as their periods of revolution about their primaries In this case, as in every other, he considered a discovery in each and every one of its possible bearings There are no instances where he has singularly overlooked the consequences of his observations