"What's that for?" the boy asked.
"So that we can look at the reefs at their own
level," was the reply. "No matter how much you allow for refraction and foreshortening, you'll find it almost impossible to get correct values by studying a reef from the top only. You know how queer a place looks in a picture that has been taken from an aëroplane?"
"Yes," the boy answered.
"That's what we've got to avoid here. We are looking down on the reefs just as an aviator looks down on a city. This glass, however, will give me the proper perspective. You see I have made it something like a telescope so that I can add segment after segment and watch conditions even in fairly deep water. Now I'll show you how I'm going to manage it."
He took the long L glass with which he was working and fastened it by little hooks to the direct overhead glass which Colin was using, and as he did so the boy noticed that the two glasses were so arranged that they focussed at the same point of the reef, only that one viewed it from above, the other from the side. A little device worked by a thumbscrew varied the angle in proportion to the depth.
"Now," he was instructed, "draw in and color—as well as you know how—everything you see
in the field of your glass. You've got all day to do it in, so there's no need for hurry. Remember, I don't want the color you think the sea-fans and other forms would be out of the water, but the color that they seem to you to be when looked at through the water."
"But I don't draw so awfully well, Mr. Collier," said Colin.
"You don't need to," was the reply, "it's the color that I want. There isn't a tint known that you can't find in those pastels and I want it as exact as you can get it. I'm going to do the same thing, you see, only from the side. The light will cause a good deal of difference, and I want to determine just how the shadows fall."