“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.