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"The canary creeper's got in among the pine trees now," said the man with the lorgnette. "It wasn't there this morning. You can see it grow while you watch it." He took out a handkerchief and wiped his object-glasses with careful deliberation. "I reckon you're going down there," ventured Skelmersdale. "Will you come?" said Cossar. Skelmersdale seemed to hesitate. "It's an all-night job."
Looking up these sponges a bird's-eye view would closely resemble the lichen-like vegetation of frost on window panes; or that vegetation in Canada-balsam which mad philosophical instrument makers will put between the lenses of the object-glasses of our telescopes.
A mile or two from here skilful artisans make those great object-glasses with which the mysteries of the stars are disclosed. The slightest speck or flaw blurs the image, but with the perfect glass stars unseen by any eye throughout the history of the world are to be in our days discovered. It is a parable of the soul.
These telescopes were of enormous lengths, three of his object-glasses, now in possession of the Royal Society, being of 123, 180, and 210 feet focal length respectively.
No European optician will claim to do better than the American firm of Alvan Clark & Sons in producing uniformly good object-glasses, and this firm always does the work by hand, moving the glass over the polisher, and not the polisher over the glass.
With equal object-glasses they have a greater magnifying power, because the luminous rays that traverse the glasses lose less by absorption than the reflection on the metallic mirror of telescopes; but the thickness that can be given to glass is limited, for too thick it does not allow the luminous rays to pass.
Very small object-glasses of one or two inches may be a little higher than would be given by this rule. The tube for the telescope may be made of paper, by pasting a great number of thicknesses around a long wooden cylinder. A yet better tube is made of a simple wooden box.
Perfect definition becomes with increasing size, more and more difficult to attain; once attained, it becomes more and more difficult to keep. For the huge masses of material employed to form great object-glasses or mirrors tend with every movement to become deformed by their own weight.
When the light was allowed to fall upon the object-glass, every different thickness of the plate of air between the object-glasses gave different colours, so that the point where the two object-glasses touched one another was the centre of a number of concentric coloured rings.
Barbicane likewise remarked the wide craters with no interior cones, which are of a bluish colour, analogous to that of fresh-polished sheets of steel. These colours really belonged to the lunar disc, and did not result, as certain astronomers think, either from some imperfection in the object-glasses of the telescopes or the interposition of the terrestrial atmosphere.
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