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Gulping a cup of tea and a few sandwiches, the three cadets continued their advance toward the uncharted, unknown dangers of the asteroid belt that lay ahead of them. Meanwhile, back on the Polaris, Jeff Marshall walked into the observatory quietly. He stood and watched Professor Sykes adjust the prisms of his telescopes, then settle himself to an hour of observation.
It being assumed that columnar trap has consolidated from a fluid state, the prisms are said to be always at right angles to the COOLING SURFACES. If these surfaces, therefore, instead of being either perpendicular or horizontal, are curved, the columns ought to be inclined at every angle to the horizon; and there is a beautiful exemplification of this phenomenon in one of the valleys of the Vivarais, a mountainous district in the South of France, where, in the midst of a region of gneiss, a geologist encounters unexpectedly several volcanic cones of loose sand and scoriae.
It is not absolutely indicative of the presence of blood, for tincture of guaiacum is coloured blue by milk, saliva, and pus. A cover-glass is placed over this, and a drop of glacial acetic acid allowed to run in. It is again heated until bubbles appear. Crystals of hæmin may now be detected by the microscope. They are dark brown or yellow rhombic prisms. These bands indicate oxyhæmoglobin.
But now I turn the film so as to render its directions of vibration oblique to the two tourmaline axes; then, you see it exercises the power, demonstrated in the last lecture, of partially restoring the light. Let us now mount our Nicol prisms, and cross them as we crossed the tourmaline.
A huge chandelier with prisms hung over the flowers at the centre of the table, which sparkled with glass and silver, while dishes of vermilion and yellow fruits relieved the whiteness of the cloth. Honora found herself beside Mr. Holt, who looked more shrivelled than ever in his evening clothes.
There was one slight mistake in Newton's belief that all prisms would give a spectrum of exactly the same length, and it was some time before he corrected this error. Meanwhile he patiently met and answered the arguments of his opponents until he began to feel that patience was no longer a virtue.
We have, first, our electric lamp, then a Nicol prism, to polarize the beam emergent from the lamp; then an electro-magnet, then a second Nicol, and finally our screen. At the present moment the prisms are crossed, and the screen is dark. I place from pole to pole of the electro-magnet a cylinder of a peculiar kind of glass, first made by Faraday, and called Faraday's heavy glass.
In moments of sober reflection our resolutions are like prisms of basalt, that will not be riven by the lightning, but which in the hour of real trial prove to be ice-crystals that a sunbeam can dissolve.
In vain Büttner pressed many times for the return of the prisms; at last he sent a mutual acquaintance with the injunction not to return without them. Goethe then searched for the long-neglected apparatus and determined to take a rapid glance through one of the prisms before he gave them back.
You have seen in these lectures a single prism employed to produce a spectrum, and you have seen a pair of prisms employed. In the latter case, the dispersed white light, being diffused over about twice the area, had all its colours proportionately diluted.
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