"I've noticed," the boy said thoughtfully, "that red is only used for the smaller lights. I wanted to ask you about that the other day. Now there's Point Adams Light," he continued, pointing off the starboard bow as the lighthouse-tender steamed out of the mouth of the Columbia River, "it looks just as big as this light on the other side, on Cape Disappointment, but it's a lot harder to see. When I've been headed for home, on a misty night, after a day's fishing, I've missed Point Adams when Cape Disappointment was as clear as could be."
"But you could see other lights?"
"Oh, yes, there wasn't any difficulty in making the harbor, either in or out. I was just wondering whether the color of the light had anything to do with making it seem dim?"
"Of course," his father answered; "a red glass cuts off sixty per cent, of the light. You can't see the Point Adams Light for more than about eleven miles, but, in ordinarily clear weather, you can see the fixed white light of Cape Disappointment for all of twenty-two nautical miles."
"I don't quite see why," said the boy, puzzled.
"That's because you're not taking the trouble to think," was the impatient reply. "You know that light is made up of all the colors of the rainbow?"
"Of course."
"And red is only a small part of that, isn't it?"
"Yes."
"Well, don't you see? Red glass only lets the red rays through and cuts off all the rest. How could it help being a lot fainter? And, what's more, red doesn't excite the nerves of the eye as much as white does, so that if there were two lights of equal power, one red and one white, the red would be less easily seen."