He is making some experiments in connection with some of his cases, and when the Doctor locks himself in to make experiments, something interesting generally follows.

I should like to know what it is this time." "I suppose there is a keyhole in the laboratory door ?" I suggested, with a grin.
"Sir!" he exclaimed indignantly.

"Dr.Jervis, I am surprised at you." Then, perceiving my facetious intent, he smiled also and added: "But there _is_ a keyhole if you'd like to try it, though I'll wager the Doctor would see more of you than you would of him." "You are mighty secret about your doings, you and the Doctor," I said.
"Yes," he answered.

"You see, it's a queer trade this of the Doctor's, and there are some queer secrets in it.

Now, for instance, what do you make of this ?" He produced from his pocket a leather case, whence he took a piece of paper which he handed to me.