He screwed up face and did complicated things with his fingers. “Twenty-fives into ones, you can’t. No, anyway you don’t want to. Drat. Wait a bit.” He opened his eyes suddenly and began to speak rapidly. “The twenty-five-minim syringe could hold a twentieth of a grain of hyoscine, and the vet’s pump could hold eleven thirty-seconds of a grain. There!” he added proudly.

“Quite correct—good for you!” shouted Thoms, clapping the inspector on the back.

“There’s more to come. I can do better than that. Eleven thirty-seconds is three thirty-seconds more than a quarter, which is only eight thirty-seconds. How’s that?”

“Brilliant, but I don’t see the application?”

“Don’t you?” asked Alleyn anxiously. “And yet I know I thought it rather important a moment ago. Ah, well—it’s gone now. I’ll just write the others down.”

Mr. Thoms moved to his elbow and looked curiously at his tiny hieroglyphics.

“I can’t see,” complained Alleyn and walked over to the light.

Mr. Thoms did not follow and so did not see the last of his minute entries, which read:

“The large syringe could hold a little over the amount found at the P.M.”

He shut his little book tenderly and put it in his pocket.

“Thank you a thousand times, Mr. Thoms,” he said. “You’ve made it very easy for me. Now there’s only one more person I’ve got to see to-day and that’s Dr. Roberts. Can you tell me where I’ll find him?”

“Well, he’s not the usual anæsthetist here, you know. He does a lot of Dr. Grey’s work for him. Hasn’t been in since this affair. I should think at this time you’d find him at his private address. I’ll ring up his house if you like.”

“That’s very good of you. Where does he live?”

“Not sure. His name’s Theodore. I know that because I heard Grey calling him Dora. Dora!” Mr. Thoms laughed extensively and led the way to a black hole with a telephone inside it.

He witched on a light and consulted the directory.

“Here we are. Roberts, Roberts, Roberts. Dr. Theodore. Wigmore Street. That’s your man.”

He dialled the number. Alleyn leant patiently against the door.

“Hullo. Dr. Roberts’s house? Is he in? Ask him if he can see Inspector—” He paused and put his hand over the receiver. “Alleyn, isn’t it? Yes—ask him if he can see Inspector Alleyn if he comes along now.”