"An interesting optical delusion," declared the professor. "I must make a note of it, and——"
"Wow! There it goes ag'in."
"Where? Where?" chorused the boys.
"Right off there! Look! Look!"
The lanky cow-puncher, fairly dancing about with excitement, pointed out beyond the shadow of the solitary mesa. Sure enough, there were three or four enormous, black, shadowy shapes, traveling across the sands at a seemingly great speed.
"Get your rifles, boys!" yelled Jack.
The weapons lay handy, and in a jiffy four beads had been drawn on the immense, vague shapes.
But even as their fingers pressed the triggers, and the four reports rang out as one, the indefinite forms vanished as mysteriously as they had appeared.