are undoubtedly parietosplanchnic Lamellibranchiates, afflicted severely with psittaceous psychopannychism.' Such nonsense must have been an effectual safety-valve for the writer's feelings. On a similar occasion another astronomer rushed from the dome-room into an adjoin- An attack. ing apartment, seized a poker, and paced furiously up and down past the base-burning stove; a fresh hole gaped in the isinglass whenever he went by.

Ordinarily, however, the director of an observatory treats visitors who come at proper times with the utmost courtesy, and is best pleased if they rain upon him a shower of questions about his instruments or the celestial objects in view.

Roger Bacon.

Hans
Lippershey.

Galileo.

Failures.

CHAPTER VIII.

A GREAT TELESCOPE.

"Through thee will Holy Science, putting off
Earth's dusty sandals from her radiant feet,
Survey God's beauteous firmament unrolled
Like to a book new-writ in golden words,
And turn the azure scroll with reverent hand,
And read to man the wonders God hath wrought."
-A. V. G.

THE great telescope of to-day has been evolved during the past three centuries by a slow process of growth. Before its actual invention many men had ideas about the possibility of making an instrument which would make distant objects appear near at hand. Roger Bacon, who died in 1294, stated that transparent bodies could be made in such forms and placed in such combinations as to magnify objects. But he never constructed a telescope, for he ascribed to such an instrument some properties which it does not possess.

In 1608 Hans Lippershey, a resident of Middleburg, Holland, invented the telescope. During the ensuing year Galileo heard of the new invention, and reinvented the instrument. He made several small telescopes, the most powerful of which magnified thirty-three diameters, and revealed the spots on the sun, the lunar mountains, the moons of Jupiter, and the phases of Venus.

Slight progress was made during the next hundred years. Men failed to get clear, sharp images of objects, no matter how accurately they ground the lenses of

their telescopes. Even the immortal Newton was foiled. Newton foiled. When he discovered that white light was dispersed

into a number of different colors by being passed Dispersion of through a prism, he also found that passage through a

light.

Newton's reflector.

Lord Rosse's reflector.

Silver on glass.

lens affected it in the same way. Believing that the dispersed rays could not be reunited, Newton gave up all hope of perfecting Galileo's form of telescope, and turned his attention to making concave mirrors, which reflected the light to a focus without dispersing it. Newton's first reflecting telescope was six inches long, and was equipped with a mirror one inch in diameter. So successful was the performance of this pigmy that he made a larger one, which is now in the possession of the Royal Society of London, and bears this inscription: "The first reflecting telescope, invented by Sir Isaac Newton, and made with his own hands."

As the years rolled on, reflecting telescopes of larger and larger sizes were made, until at last Lord Rosse's leviathan, which has a mirror six feet in di

ameter, was mounted at Parsonstown, Ireland, fifty years ago. No other reflector of equal size has yet been constructed. Its mirror was made of polished metal. It is now customary to make the mirror of glass, and to coat it with silver.

Such telescopes offer special advantages for photographic and spectroscopic work, since the light which impinges upon a mirror suffers no dispersion, as it would if passed through a lens.

Since reflectors are little used in this country, we return to the history of the common form of telescope, A refractor. which is called a refractor. The name refractor arises from the fact that rays of light, in passing through a lens, are bent, or "refracted."

We have noticed that Newton thought it impossible to reunite the rays of various colors which were scattered in passing through his lenses. But early in the eighteenth century a well-to-do countryman of his, Mr. Chester Moor Hall, was struck with the fact that the human eye, which contains more than one refractive medium, produces images practically free from obnoxious color fringes. By combining two lenses of different kinds of glass he reunited the dispersed rays pretty well. Being a gentleman of leisure, he took no particular pains to follow up his discovery, and the credit of it was soon given to Mr. John Dollond, an optician, who experimented successfully along similar lines and published an account of his work in 1758.

Color fringes.

A new diffi

A new difficulty of the first magnitude now arose. Good discs of glass more than three and a half inches in culty. diameter could not be procured. In vain the French Academy offered prizes for larger discs; the best chemists were baffled. But the battle is not always to the strong. From 1784 to 1814, Guinand, a poor Swiss watchmaker, toiled with dauntless industry, overcoming one obstacle after another, until he succeeded in producing glasses eighteen inches in diameter.

The manufacture of a large disc of optical glass * requires the utmost carefulness, as well as a high degree

*There are now only three firms in the world which have made very large lenses, Chance & Co., of Birmingham, Mantois of Paris, and Schott & Co., of Jena. Schott & Co. now produce a number of different kinds of glass, and a large amount of experimentation is going on, in an endeavor to find combinations of lenses which will give more satisfactory results than the time-honored combination of a lens of crown glass backed up by another one of flint glass. Professor Hastings, of Yale, has been successful in such researches.