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As six thousand stars can be seen by the eye all over the heavens, we may fairly expect that twenty times that number that is to say, one hundred and twenty thousand stars could be shown by the opera-glass over the entire sky. Let us go a step further, and employ a telescope, the object-glass of which is three inches across.
An object-glass for which he had furnished the material to Cauchoix, procured him, in 1823, a royal invitation to settle in Paris; but he was no longer equal to the change, and died at the scene of his labours, February 13 following. Bequeathed by South to Trinity College, Dublin, it was employed at the Dunsink Observatory by Brünnow and Ball in their investigations of stellar parallax.
Lydgate talked persistently when they were in his work-room, putting arguments for and against the probability of certain biological views; but he had none of those definite things to say or to show which give the waymarks of a patient uninterrupted pursuit, such as he used himself to insist on, saying that "there must be a systole and diastole in all inquiry," and that "a man's mind must be continually expanding and shrinking between the whole human horizon and the horizon of an object-glass."
We should not, it is true, dare to assert that any ingenious young man, with a clear appreciation of optical principles, could not soon learn to grind and polish an object-glass for himself by the method we have described, and thus obtain a much better instrument than Galileo ever had at his command.
The clock consisted of a spy-glass, having a nichol or double-image prism for an eye-piece, and a thin plate of selenite for an object-glass. When the tube was directed to the North Pole that is, parallel to the earth's axis and the prism of the eye-piece turned until no colour was seen, the angle of turning, as shown by an index moving with the prism over a graduated limb, gave the hour of day.
All the difficulty in the execution of these optical instruments lies in the making of the object-glass, whether they be made of glass or metallic mirrors. Still at the epoch when the Gun Club made its great experiment these instruments were singularly perfected and gave magnificent results.
When this cap is removed the slit appears as a narrow vertical band with much fainter bands on both sides of it. Cover one of the holes, and the fringes instantly disappear. Their production requires the joint effect of the two light-pencils. Now suppose the two holes over the object-glass to be in movable plates, so that their distance apart can be varied.
Then comes a description of the construction of the object-glass, twenty-four feet in diameter, 'just six times the size of the elder Herschel's; who, by the way, never made a telescope with an object-glass.
In the centre of the solid floor, levelled to the utmost perfection, was left a circular pillar supporting the polar axis of an instrument widely differing from our telescopes, especially in the fact that it had no opaque tube connecting the essential lenses which we call the eye-piece and the object-glass, names not applicable to their Martial substitutes.
The use of a slit was discarded as unnecessary for objects like the stars, devoid of sensible dimensions, and giving hence a naturally pure spectrum; and a large prism, placed in front of the object-glass, analysed at once, with slight loss of light, the rays of all the stars in the field. Their spectra were taken, as it were, wholesale.
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