Sir William Herschel: His Life and Works

by Edward S. Holden

Researches on the Variable Emission of Light and Heat from the Sun.

It is certainly a remarkable fact that Herschel was the first observer to recognize the real importance of the aperture or diameter of a telescope. Before his time it was generally assumed that this element only conditioned the amount of light transmitted to the eye, or, in other words, merely determined the brightness of the image. Hence the conclusion that if an object is sufficiently bright, the telescope may be made as small as desired without loss of power. Thus, in observing the sun, astronomers before Herschel had been accustomed to reduce the aperture of their telescopes, in order to moderate the heat and light transmitted. Scheiner, it is true, nearly two centuries before the time we are considering, had invented a method for observing the sun without danger, still employing the full aperture. This was by projecting the image of the sun on a white screen beyond the eye-piece, the telescope being slightly lengthened. For special purposes this ingenious method has even been found useful in modern times, though for sharpness of definition it bears much the same relation to the more usual manner of observing, 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 way were very great. We have noted his attempts to find screens which would effectively cut off a large portion of the heat and light without impairing vision, and have considered, somewhat in detail, the remarkable discoveries in radiant heat to which these attempts led him. His efforts were not unsuccessful. A green glass smoked, and a glass cell containing 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 time, there has been but a single 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 times and for the indefatigable observer Schwabe. The probable importance of such a period in its relation to terrestrial meteorology 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 meteorological 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 mind, though it could not recall lost opportunities for solar observations, did find a substitute for meteorological records in the statistics of the prices of grain 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 method, as ingenious 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 Nebulæ and Clusters.

When Herschel first began to observe the nebulæ in 1774, there were very few of these objects known. The nebulæ 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 comets, and twenty-eight nebulæ 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 nebulæ, 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 nebulæ 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 nebulæ 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 nebulæ now known the Herschels discovered at least eight-tenths. The mere discovery of twenty-five hundred nebulæ would have been a brilliant addition to our knowledge of celestial statistics.