The Vast Abyss

“There’s nothing the matter, uncle, only I’m a bit puzzled.”

“What about?”

“Over this great glass. It’s going to be so different to the old one.”

“Of course; that is a refractor, and this is going to be a reflector.”

“Yes, uncle, but it seems so queer. The refractor is a tube made so that you can look through it, but the reflector will be, if you are right, so that you can’t look through it, because instead of being at the end, the hole will be in the side. Is that correct?”

“Quite right, and you are quite wrong, Tom, for you do not understand the first simple truth in connection with a telescope.”

“I suppose not, uncle,” replied the lad, with a sigh. “I am very stupid.”

“No, you are not, sir, only about as ignorant as most people are about glasses. I have explained the matter to you, but you have not taken it in.”

“I suppose not, uncle,” said Tom, wrinkling his brow.

“Then understand it now, once for all. It is very simple if you will try and grasp it. Now look here: what do you do with an ordinary telescope or opera-glass, single or double? Hold it up to your eyes, do you not?”

“Yes, uncle.”

“And then?”

“Look through it at something distant, and it seems to draw it near.”

“You do what?”

“Look through it, uncle.”

“Nothing of the kind, sir, you do not.”

Tom looked puzzled. What did his uncle mean? He had, he thought, looked through a pair of field-glasses scores of times at home in the old days.

“I make you stare, my lad, but I am glad to see it, for it shows me how right I am, and that you do think as everybody else does who has not studied optics, that you look through a glass at an object.”

Tom stared harder, and once more the old idea came to him, and he asked himself whether there were times when his uncle did not quite understand what he was saying.

“But you do, uncle,” he cried at last. Then he qualified this declaration by saying, “Don’t you?”

“No, my boy, once for all you do not; and if you take up any telescope, and remove the eye-piece before looking along the tube, you will see that your eyes will not penetrate the glass at the end. Then if you try the eye-piece alone, you will find that you cannot even look through that. How much less then will you be able to look through both at once.”

“But it seems so strange, uncle. You have a big magnifying-glass in a tube, and don’t look through it? Then what do you do?”

“Certainly not look through it, my boy.”

“But the bigger the glasses are the more they magnify – the moon, say.”

“Yes, Tom; and the more light they gather.”

“Well, then, why do you say, uncle, that you don’t look through the glass?”

“Because it is a fact that I want you to understand,” said Uncle Richard, smiling. “The big glass, or in our case the reflecting speculum, forms a tiny image of the object at which it is pointed, close to where we look in, within an inch or so of our eye.”

“A tiny image, uncle?”

“Well, picture, then.”

“But you say tiny! It looks big enough when we put our eye to the little round hole.”

“To be sure it does. But what do you look through?”

“The eye-piece.”

“Well, what is the eye-piece?”

“A little glass or two – lenses.”

“These glasses or lenses form a microscope, Tom; and through them you look at the tiny image formed in the focus of the great lens or the speculum, whichever you use.”

“But I thought microscopes were only used to magnify things invisible to the eye.”

“Well, Jupiter’s moons, Saturn’s ring, and the markings on Mars are all invisible to the naked eye. So are the craters in the moon; so we use the big speculum to gather the light, and then look at the spot where all the rays of light come to their narrowest point, with an eye-piece which really is a microscope.”

“But I don’t understand now,” said Tom uneasily. “I wish I was not so – ”

“If you say stupid again, Tom, I shall quarrel with you,” said Uncle Richard sternly. “I never think any boy is stupid who tries to master a subject. One boy’s brain may be slower at acquiring knowledge than another, but that does not prove him to be stupid. What is it you don’t follow?”

“About our telescope. If the light from the big speculum is all reflected nearly to a point, ought we not to look down at it?”

“No; because then our heads would be in the way, and would cast a shadow upon it. To avoid that, I put the little mirror in the middle, near the top, just at the right slant, so that the rays are turned off at right angles into the eye-piece, and so we are able to look without interrupting the light.”

“Oh, I see now,” said Tom thoughtfully. “It’s very clear.”

“Yes,” said Uncle Richard. “Sir Isaac Newton, who contrived that way, was a clever man. Now then, let’s get on with our work.”

“I suppose then now we’re ready?” said Tom.

“Far from it,” replied his uncle; “are you going to hold up a twelve-foot tube to your eye, and direct it to a star? The next thing is of course to mount it upon trunnions, and arrange that it shall turn upon an axis, so that we can sweep in any direction.”

The longest tasks come to an end. By the help of the village carpenter, a strong rough stand was connected with the beam formerly used to bear the sails of the mill, the trunnions were fitted to a strong iron ring by the smith, and one evening the great telescope was hung in its place, and in spite of its weight, moved at the slightest touch, its centre of gravity having been so carefully calculated that it swung up and down and revolved with the greatest ease.

“There, Tom,” said Uncle Richard; “now I think we can sweep the heavens in every direction, and when once we have tried, the mirrors, so as to set them and the eye-piece exact, we can get to work.”

Tom looked at his uncle in dismay.

“Why, you don’t mean to say, uncle, that there is more to do after working at it like this?”

“Yes, a great deal. We have to get the glasses to work with one another to the most perfect correctness. That task may take us for days.”