"In other words, an invisible light?" "Yes. That is, invisible to the human eye. The effect of this beam of ultra-violet light in the form of severe sunburn would be readily apparent if you exposed your skin to it for any length of time, and the effects on your eyesight of continued gazing would be apt to be disastrous. It would produce a severe opthalmia and temporary impairment of the vision, somewhat the same symptoms as are observed in snow blindness." "I see. May I ask what is the object of the whole thing?" "Surely. Before we can successfully combat this peculiar visitant from another world, it is necessary that we gain some idea of the size and appearance of it. Nothing of the sort has before made its appearance, so far as the annals of science go, and so I am forced to make some rather wild guesses at the nature of the animal. You are probably aware of the fact that the property of penetration possessed by all waves is a function of their frequency, or, perhaps I should say, of their wave-length?" * * * * * "Certainly." "The longer rays of visible light will not penetrate as deeply into a given substance as the shorter ultra-violet rays. This visitor is evidently from some unexplored and, indeed, unknown cavern in the depths of the earth where visible light has never penetrated. Apparently in this cavern the color of the inhabitants is ultra-violet, and hence invisible to us." "You are beyond my depth, Doctor." "Pardon me. You understand, of course, what color is? When sunlight, which is a mixture of all colors from infra-red to ultra-violet inclusive, falls on an object, certain rays are reflected and certain others are absorbed. If the red rays are reflected and all others absorbed, the object appears red to our eyes. If all the rays are reflected, the object appears white, and if all are absorbed, it appears black." "I understand that." "The human eye cannot detect ultra-violet. Suppose then, that we have an object, either animate or inanimate, the surface of which reflects only ultra-violet light, what will be the result? The object will be invisible." "I should think it would be black if all the rays except the ultra-violet were absorbed." "It would, but mark, I did not say the others were absorbed. Are you familiar with fluorescein?" "No." "I think you are. It is the dye used in making changeable silk. If we fill a glass container with a fluorescein solution and look at it by reflected light it appears green. If we look at it by transmitted light, that is, light which has traversed the solution, it appears red. In other words, this is a substance which reflects green light, allows a free passage to red light, and absorbs all other light. This creature we are after, if my theory is correct, is composed of a substance which allows free passage to all of the visible light rays and at the same time reflects ultra-violet light. Do I make this clear?"