Page:A Voyage in Space (1913).djvu/154

134 knowledge we got simply by watching more carefully with a new kind of photometer invented by Mr. Joel Stebbins in America. The principle of it can easily be illustrated by an experiment. There is a substance called selenium, which is sensitive to light, so that when light falls upon it the resistance alters, and that spot of light on the screen will move. The selenium is shut up in this box, so that light shall not disturb it, otherwise the spot would be moving about instead of remaining steady. But now we will darken the room and open the box: so long as the box remains still in the dark the spot remains steady; but now we shine a taper on the selenium and you see the spot move off at once. You will understand that by using this principle, Mr. Stebbins was able to tell when a star was shining on his selenium and also how bright the star was: and though he had many difficulties at first, he got over them so successfully that in the end the instrument could tell the brightness of the star much better than the human eye could. I will mention just one of his difficulties. He found that the instrument behaved differently in warm weather and cold, so that to get consistent results he determined to keep it always at the freezing temperature by packing it in ice. In America there is always plenty of ice to be got, but it is generally used for cooling drinks: and when Mr. Stebbins sent in his bill for all the ice wanted to keep his photometer cool, the authorities pretended to think that he must have required a great many drinks. But Mr. Stebbins was so determined to make his photometer work that he bore even practical jokes in the good