Part 12 (2/2)

It is certainly a renize the real importance of the aperture or diaenerally assuht transmitted to the eye, or, in other words, e Hence the conclusion that if an object is sufficiently bright, the telescope may be made as s the sun, astronomers before HERSCHEL had been accustomed to reduce the aperture of their telescopes, in order to ht transmitted SCHEINER, it is true, nearly two centuries before the ti the sun without danger, still ee of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened For special purposes this ingenious h for sharpness of definition it bears , that a photographic picture does to direct vision

Although HERSCHEL saw the advantages of using the whole aperture of a telescope in such observations, the practical difficulties in the ere very great We have noted his attee portion of the heat and light without i vision, and have considered, somewhat in detail, the remarkable discoveries in radiant heat to which these attereen glass s a solution of black writing ink in water--were found to work admirably

Thus provided with more powerful instrumental means than had ever been applied to the purpose, HERSCHEL turned his attention to the sun In a very short time he exhausted nearly all there was to be discovered, so that since the publication of his last paper on this subject, in 1801, until the present tile telescopic phenomenon, connected with the physical appearance of the sun, which was unknown to HERSCHEL That phenomenon is the frequent occurrence of a darker central shade or kernel in large spots, discovered by DAWES about 1858

HERSCHEL, though observing a hundred and ninety years after the earliest discovery of sun spots, seems to have been the first to suspect their periodic character To establish this as a fact, and to measure the period, was left for our own tiable observer SCHWABE The probable importance of such a period in its relation to terrestrial y was not only clearly pointed out by HERSCHEL, but he even attempted to demonstrate, from such data as were obtainable, the character of this influence

Perhaps no one thing which this great philosopher has done better exhibits the catholic character of his mind than this research When the possible connection of solar and terrestrial phenomena occurred to him as a question to be tested, there were no available ical records, and he could find but four or five short series of observations, widely separated in time To an ordinary thinker the task would have seemed hopeless until more data had been collected But HERSCHEL'S fertile h it could not recall lost opportunities for solar observations, did find a substitute for rain during the various epochs

It is clear that the price of wheat must have depended upon the supply, and the supply, in turn, largely upon the character of the season

The enious as it is, failed in HERSCHEL'S hands on account of the paucity of solar statistics; but it has since proved of value, and has taken its place as a recognized method of research

_Researches on Nebulae and Clusters_

When HERSCHEL first began to observe the nebulae in 1774, there were very few of these objects known The nebulae of _Orion_ and _Andromeda_ had been known in Europe only a little over a hundred years

In 1784 MESSIER published a list of sixty-eight such objects which he had found in his searches for coht nebulae had been found by LACAILLE in his observations at the Cape of Good Hope In the mere discovery of these objects HERSCHEL quickly surpassed all others

In 1786 he published a catalogue of one thousand new nebulae, in 1789 a catalogue of a second thousand, and in 1802 one of five hundred In all he discovered and described two thousand five hundred and eight new nebulae and clusters This branch of astronomy may almost be said to be proper to the HERSCHELS, father and son Sir JOHN HERSCHEL re-observed all his father's nebulae in the northern hemisphere, and added many new ones, and in his astronomical expedition to the Cape of Good Hope he recorded almost an equal number in the southern sky

Of the six thousand two hundred nebulae non the HERSCHELS discovered at least eight-tenths The mere discovery of twenty-five hundred nebulae would have been a brilliant addition to our knowledge of celestial statistics

HERSCHEL did more than merely point out the existence and position of these new bodies Each observation was accompanied by a careful and minute description of the object viewed, and with sketches and diagraave the position of the small stars in it and near it[36]

As the nebulae and clusters were discovered they were placed in classes, each class covering those nebulae which reseeneral features Even at the telescope HERSCHEL'S object was not discovery merely, but to know the inner constitution of the heavens

His classes were arranged with this end, and they are to-day adopted

They were:

CLass I ”Bright nebulae (288 in all)

II ”Faint nebulae (909 in all)

III ”Very faint nebulae (984 in all)

IV ”Planetary nebulae, stars with burs, with milky chevelure, with short rays, ree nebulae (52 in all)

VI ”Very compressed and rich clusters of stars (42 in all)

VII ”Pretty much compressed clusters (67 in all)

VIII ”Coarsely scattered clusters of stars” (88 in all)

The lists of these classes were the storehouses of rich material from which HERSCHEL drew the examples by which his later opinions on the physical conditions of nebulous matter were enforced

As the nebulae were discovered and classified they were placed upon a star-map in their proper positions (1786), and, as the discoveries went on, the real laws of the distribution of the nebulae and of the clusters over the surface of the sky showed themselves more and more plainly It was by this means that HERSCHEL was led to the announcement of the law that the spaces richest in nebulae are distant from the Milky Way, etc

By no other means could he have detected this, and I believe this to have been the first exaraphical e masses of statistics