Sir James Lubbock unlocked another cupboard, and produced a large, heavy instrument, rather like a telescope fixed to a stand. He put it down beside the two bottles and departed in search of water. While he was preparing solutions from the respective bottles of muscarine, Waters turned to me.
“You’d better have this quite clear in your mind—I mean, you’d like to know what you may expect to see, exactly.”
“Yes,” I said. “At present I feel rather like the good lady in The Moonstone , who wanted to know when the explosion would take place.”
“I’m afraid it won’t be so exciting as that. Cheer up, man, you look as white as a sheet. At the further end of the instrument is a thin plate of the semitransparent mineral, tourmaline. You’ve seen it in jewellers’ shops. Pretty stuff, and all that, and, what is more to the purpose, it has a very finely foliated structure. In a ray of ordinary light, the vibrations take place in all directions, but when passed through a slice of tourmaline they are confined to one plane, and the light is then polarised. We talked about that at dinner—you remember. This slice of tourmaline is called the polariser. Right. Now at this end, near the eyepiece, is a second slice of tourmaline, which can be rotated, and which is called the analyser. Now, when the analyser is turned so that its foliations are parallel to those of the polariser, light will pass through both, but if the analyser is turned so that its foliations are at right angles to those of the polariser, then no light will pass and there will be darkness. All clear so far?”
“Perfectly.”
“Very well. Now, if, when the analyser is thus turned to darkness, I place the solution of an optically active substance between the two slices of tourmaline the light will—you can tell me that yourself—it’s a band of light, remember.”