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DEFECT OF ITS CONSTRUCTION.

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lens was plain, or flat, on one side, and convex on the other. At the end of the tube nearest to the eye, he fixed another lens, plain on one side, and concave on the other.

I told you, in the last Lecture, that rays, after passing through a convex lens, are refracted, or change their direction, drawing nearer and nearer to each other, till they meet in the focus. Galileo placed his concave eye-glass, so as to intercept the converging rays proceeding from the object-glass, before they reached the focus; in passing through the concave lens, their direction was again changed; the rays no longer converged, but diverged and passed onwards to the eye of the spectator. This telescope had considerable power, but did not magnify so much as if the eye glass had been a convex lens. Another inconvenience was produced by the divergence of the rays:-you know the pupil of the eye is a small aperture, or opening, and many of the rays were spread too widely to enter the pupil. The consequence of this was, that with a telescope so constructed, the spectator

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GALILEO'S DISCOVERY.

could command only a small space. The space,

or area, that can be surveyed at once through a telescope, is called, "the field of view." Galileo's field was a small one; he was therefore obliged frequently to shift his telescope; and later philosophers may wonder at the patience and address he must have exercised, to make the discoveries by which his perseverance was so gloriously rewarded.*

A third inconvenience resulted from the construction of Galileo's telescope. Every object seen through it appeared to be inverted, like the phantoms with which we amused ourselves in the last Lecture. Men, churches, trees, all were seen upside-down. This was a very awkward circumstance in looking at terrestrial objects, but not of much consequence to Galileo, who desired, above all things, to apply his new invention to the observation of the stars.

What words can express the delight he felt, when he first directed his improved telescope,— that which magnified thirty-three times,-to the

Hutton, ib. Arnott, ii. 295.

PHASES OF VENUS.

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heavens, and through it contemplated so many glorious objects, which no human eye had ever before seen! There he beheld Venus, no longer appearing only as a beautiful morning and evening star, but like another moon-first a slender crescent,

"Like a silver bow,

New strung in heaven, she lifts her beamy horns;"

When

then passing onward in her orbit, the crescent became broader, then a half circle, till at length its whole illuminated surface being opposite to the earth, it presented the appearance of a moon at the full. Galileo might previously have supposed this was likely to be true, but now he saw it with his own eyes, as distinctly as he could observe the changes of the moon. Venus is between the earth and the sun, she becomes invisible to the inhabitants of our planet, just as the moon does when in the same situation, because their dark sides are next to us. It is then new Moon, and new Venus. When the moon is so circumstanced, it sometimes hides a part or the whole of the sun's

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TRANSIT OF VENUS.

disk, and this we call an eclipse of the sun. The same thing occasionally happens with Venus, but she is at so great a distance, that we cannot see it without the help of a telescope: through that instrument she has been observed passing over the sun's face, like a little black spot. This is called the transit of Venus:-transit, transition, and other words of the same kind, come from the Latin, and express a passing over, removal, change of state, or place.

I cannot now explain to you the important discoveries which have been made by observing the transit of Venus: it has enabled later astronomers to determine, with great accuracy, the distance of the earth from the sun; and having discovered this, they easily found the distances of the other planets. As soon as you know the rule of three perfectly, you may go on to the higher parts of arithmetic; and when you can extract the cube-root, you may make these calculations yourselves.* I mention this, because many young ladies think, that if they were to

Joyce, Sci. Dial. ii. 203. 206.

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go beyond the first rules of arithmetic, such knowledge would be of no use to them. You certainly will not want it to reckon the value of so many yards of ribbon or muslin; but if you delight in contemplating the works of Providence, surely you must feel pleasure in every improvement of the faculties, which may enable you to form some faint conception of the laws by which the universe is governed. You will then no longer see "the moon walking in brightness," or the stars appearing one after another in the evening sky, without experiencing something of that sweet and solemn feeling, which led the Psalmist to exclaim, "The heavens declare the glory of God, and the firmament showeth his handy work." These testimonies of his power and goodness are not shown here to one, and there to another; "There is no speech nor language where their voice is not heard." Let our situation in life be what it may, to all who are blessed with a good education, the great volume of nature is opened:-we all know enough to enable us to study it more; and every page

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