I know. If the light be one hundred times less, the star will be ten times further off, for the square of ten is one hundred. I can understand now that a thing is only seen by the help of light. I do not see many stars, because their light is too little for my eyes to take in. The telescope has bigger eyes to take in the light of the distant stars and nebulæ.
The pupil of the eye is but one-eighth of an inch in diameter. An object glass of twelve inches diameter is, therefore, ninety-six, or say one-hundred times as long. As the light seen is according to the square of the diameter, the telescope of twelve inches will receive one hundred times one hundred, or ten thousand times more light.
But is there a way of measuring the quantity of light?
There is. We find that the sun has twenty-two thousand millions of times more light than the nearest of the fixed stars.
Then, the square root of this ought to tell how much further off it is. Let me see. It will be about 150,000.
Yes. If the sun were put back 150,000 times further than it is, it ought to look as brightly as that star. If it does not, it is because it is really smaller than the star.
What! 150,000 times 95 millions!
But that is nothing; for it is only to the first rank. What of the twentieth magnitude?
Yes. But you say the nebulæ are further off than that.
I may tell you that if the sun moved three times as fast as the world does, in its six hundred millions of miles a year, it would take two hundred and fifty millions of years to get to as far as Lord Rosse’s telescope could see.