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The Vast Abyss, a fiction by George Manville Fenn |
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Chapter 15 |
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_ CHAPTER FIFTEEN. Tom kept his word, for he started into wakefulness in the grey dawn out of an uncomfortable dream, in which he had seen the unfinished speculum fall off the bench on to the stone-floor, roll like a wheel out of the door, down the slope to the gate, bound over, and then go spinning down the lane and across the green, straight for the ragstone churchyard wall, where it was shivered to pieces. "Only a dream," he said, as he leaped out of bed, ran to the window, and saw by the church clock that it was only half-past four. "Time to go over and see if it is all right," he said, as he finished dressing, "and then come back and call uncle." Going down-stairs, he took the keys of the mill from where they hung by the front door, went out into the garden, unlocked the gate, and went across to the mill, where, on peering through the window, he could see the glass lying just as it had been left. "That's all right," said Tom; and he walked round by the back of the tower to see how the flowers and shrubs looked, when, to his startled surprise, he found footprints made by a heavy, clumsy pair of boots on the border beneath the wall. Their meaning was plain enough. Some one had walked along there, and got out of the yard over the wall, while, upon a little further search, he found the spot where whoever it was had entered the yard by jumping down, the prints of two heels being deeply-marked in the newly-dug earth. "That must have been Pete," said Tom, flushing; and he looked over the wall, half expecting to see the slouching figure of the lad. But there was no one within sight, and he looked round the yard in search of the visitor's object. There was nothing but the old millstones stealable, and they stood here and there where they had been leaned against tower and wall; and at ten minutes to five, after noting that the sun was shining brightly, Sam went back to his uncle and called him, and at half-past five they went together to the mill-yard, where the footprints were pointed out. "Have to keep the door carefully locked, Tom," said Uncle Richard. "Hah! capital! the sun will be shining right through that window in a few minutes." They entered the workshop, where a bench was drawn opposite to the last window, and about twelve feet away. To this, with Tom's help, the partly-polished speculum was borne. "Not very bright for a reflector, Tom," said Uncle Richard. "What am I to do to make it brighter?" "Go on polishing, uncle." "Ah, but I want to test it this morning, to see if we have a good curve," said Tom's elder, smiling. "Fill the sponge with clean water and bring it here." This was done, and the finely-ground surface was freely wetted, with the effect that it became far more luminous directly. "Now, Tom," said his uncle, "I'm going to show you something in reflection. The sun is not quite high enough for the speculum, so give me that piece of looking-glass." This was handed to him, and he held it on high, so that the low-down sun shone into it, and a reflection was cast from it back upon the wall just above the window. "See that?" "Yes, uncle. Done that many a time. Used to call it making jack-o'-lanterns." "Well, that is the effect of a reflection from a flat or plane surface; the rays of light strike back at the same angle as they hit the surface. Now then, I'll show you what happens from a curved surface." He passed the sponge rapidly over the ground speculum again, so as to glaze it--so to speak--with water, raised it upon its edge with the carefully-ground face directed at the window just as the sun rose high enough to shine in; and then by turning the great mirror slightly, the light reflected from it struck upon the wall at the side of the window. "Now, Tom, what do you see?" "A round spot of light about as big as a two-shilling piece," said the boy. "Yes; all the rays of light which fall upon our mirror, gradually drawn together to where they form an image of the sun. It is only dull, my boy, but so far finely perfect, and we can say that we have gone on very successfully." As he spoke he laid the mirror down upon its back. "Is that all you are going to do?" asked Tom. "Yes; I can test it no better till it is more advanced, my boy. It may seem a little thing to you, but it is enough to show me that we may go on, and not begin our work all over again. Now for a good turn until breakfast-time. Two good hours' work ought to produce some effect." The lower disc, now become convex, was wetted and lightly touched over with number five emery, which seemed soft enough for anything; the well-advanced mirror was turned over upon it, fitting now very closely, and with the sweet morning air floating in from the pine-woods, and the birds singing all around, the monotonous task went on with its intermissions till Uncle Richard gave the final wash off, and said--"Breakfast!" They were so far advanced now that Tom was as eager to recommence as his uncle, and by that evening so much progress had been made that the setting sun was made to shine in upon it, to be reflected back in a bright spot on the wall without the aid of water; while two evenings later, when the great round glass was stood all dry the polish upon it was limpid, and seemed to be as pure as could be. There was not the faintest scratch visible, and Tom cried in triumph-- "There, now it is done! Oh, uncle, it is grand!" "Grand enough so far, my boy. We have succeeded almost beyond my expectations; but that is only the first stage." "First--stage?" faltered Tom, looking at his uncle aghast. "Yes, boy; we have succeeded in making a beautiful spherical concave mirror, which could be of no use whatever for my purpose." "Then why did we make it?" cried Tom. "For practice?" "No, boy; because it is the step towards making an ellipse, or, as they call it when shaped for a reflecting telescope, a parabola. You know what an ellipse is?" "Gooseberry," said Tom bluntly. "Gooseberry-shaped," said his uncle. "Well then, what is a parabola?" "One of those things we used to learn about in geometry." "Good. Well, to-morrow we must begin polishing, or rather I must, to turn our glass from a spherical-curved mirror into a parabola." "You'll let me help, uncle?" "As much as I can, my boy; but the amount I have to polish off, in what is called figuring, is so small that it requires the most delicate of treatment, and first of all we have to prepare a small polisher to work by hand." This was formed of lead in the course of the next day--a nearly flat but slightly convex disc, with a handle upon its back, and when made perfectly smooth it was covered with hot pitch, which, as it cooled, was made to take the exact curve of the nearly finished mirror, by being pressed upon it, the pitch yielding sufficiently for the purpose. This done the pitch was scored across and across, till it was divided into squares, with little channels between them, so that the polishing powder and water might run freely between; then a final pressure was given upon the mirror and the implement was left to harden till the next day. "Now for a few hours' polishing," said Uncle Richard the next morning, as he took up the curved pitch tool and moistened it, no longer with emery, but with fine moistened rouge; "and if I am successful in slightly graduating off the sides here, and flattening them in an infinitesimal degree, we shall have a good reflector for our future work." But upon testing it the result that evening was not considered satisfactory. There were several zones to be corrected. It was the same the next day, and the next. But on the fourth Uncle Richard cried "Hold: enough! I think that is as good as an amateur can make a speculum, and we'll be content." That night Tom slept so soundly that he did not dream till morning, and then it was of the sun resenting being looked at, and burning his cheek, which possessed some fact, for the blind was a little drawn on one side, and the bright rays were full upon his face. "All that time spent in making the reflector!" thought Tom; "and all that work. I wonder what the next bit will be." _ |
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