Then the four lamps were exhibited, green, red, white, and blue, and the mistakes made by both essayists kept the platform in a roar. The colonel did tell which was the white--but it was probably more of a guess than a certainty. They could distinguish a "difference," they said, between two or more colours when exhibited at once, but were unable to state what that difference was. Both of them were honestly anxious that the test should be fully carried out, and answered to the utmost of their ability. Various colours were exhibited, sometimes two of nearly the same shade: it all came to the same. Long before the experiment came to an end, the fact had been fully established that both Colonel West and Matthew Cooper laboured under the defect of colour-blindness. "Cooper," said Oliver Jupp, in a good-natured tone, "they must never make an engine-driver of you again." "Well, I don't know, sir," returned Cooper, who seemed very chapfallen, "if it's true what this strange gentleman says, why--I suppose it is true. But I hope they'll make something else of me; I know I am keen enough at most things. If a man is deficient in one line, he may be all the quicker in another." "Now you have given utterance to a truism, without perhaps knowing it," interposed the professor, cheerily. "Be assured that where a defect does exist, it is amply made up for by the largeness of some other gift. Never fear that an intelligent man, like you, will want employment, because you are found not suited to the one they placed you on." "About the worst they could have given him, as it turns out," remarked Oliver Jupp, as he stood aside with the professor out of the hearing of others. "An engine-driver ought, of all men, to be able to distinguish colours." "There are some of our engine-drivers who do not, though," was the reply, as the professor cautiously lowered his voice. "Several of our worst accidents have occurred from this very fact." "Do you think so?" "I know it. It is a more frequent defect than would be thought, this absence of the organ of colour, but it is one to which little attention has been hitherto given; a subject that with some excites ridicule. A company engaging an engine-driver would as soon think of testing his capacity for eating a good dinner, as that of being able to distinguish signal lights. Most essentially necessary is it, though, that drivers, present or future, should undergo the examination." "It seems so to me," said Oliver. "And always will seem so--after this night's experiment."