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."

"Why do the railroads use red for danger signals, then?"

"Habit, mainly. It's wrong, of course, and a good many of the railroads
are changing their danger signals from red to yellow. So far as we're
concerned in the Lighthouse Service, however, we're getting rid of all
the fixed red lights wherever a long-range warning is needed."

"How do you distinguish the different lights, then?"

"Using flashing lights, with flashes of different duration."

"Why didn't you always do that?" asked Eric.

"Didn't know enough," was the simple reply. "It's only lately that
we've found out how to work a flashing light without any loss of power.
In the old days we used to depend on occulting lights, but now, flashing
lights are much more powerful. You know the difference?"

"Sure! An occulting light means that some of the time the light is shut
off, and at others it isn't. Wasn't it worked by a revolvin