"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?"
"Perfectly."