Holmes turned to me. ‘The monograph I was writing – remember Watson, I have recently become interested in stage illusions – explained it all. The Geissler tubes are filled with various gases. When an electrical current passes through them, they light up and glow in the most ethereal and glorious manner! Pass your hand near these and the lights will move! Over there, that Ruhmkorff coil—’ he pointed to that strange black tube of about three feet long and one foot in diameter, set with a series of switches and wires ‘—it creates the spark and generates the current. A smaller one was at Baker Street, Watson. I jotted some notes on this!’

I nodded.

‘I find it remarkable that I did not see you pocket those notes, Madame,’ said Holmes.

She patted the front of her dress. ‘To fool the fooler is an art you know well.’ She smiled. ‘Watson’s keys?’

He laughed.

‘I struggle for such an idea when I see Cosimo’s work. But I see instantly that your ideas complete the circle,’ said the lady. ‘I call this ‘Lucifer’s Lights’.

To my view, this was all beside the point. I glanced over at Leo Vitale. He had gone white and was staring at the floor, supporting himself against one of the laboratory tables. The girl he loved had been murdered, and here was Holmes babbling about a magic act. I pulled out a wooden stool nearby and gently sat Vitale on it, fearing a collapse.

‘Good thinking, Watson. I am sorry, Mr Vitale,’ said Holmes.

‘I was right about you, Mr Sherlock Holmes,’ said Fortuny. ‘The Bunsen burner in your sitting-room …’ He laughed.

Leo Vitale groaned. I turned to attend him but even the younger scientist seemed now to have ventured down this rabbit hole. ‘Oh, Cosimo!’ he exclaimed. ‘You debase, you subvert, you prostitute our efforts!’

‘No, Leo. Madame Borelli donates to the Cavendish Laboratory,’ said Fortuny. ‘I do this for funding.’

Vitale looked from Fortuny to Madame and back again. ‘No, Cosimo. You do this for a woman!’

Fortuny grinned at his younger associate. ‘You are in love yourself, Leo. Did you think I did not notice?’

At this, Leo Vitale seemed to crumple, and he covered his face with his hands.

‘Gentlemen, please,’ I said. ‘The lady in question has met a violent end. We are in the midst of investigating, and the police will surely come here. Holmes!’

I became aware of noises coming down the hall. As did my friend.

‘They are here!’ he cried. ‘We have wasted valuable time. Fortuny, be quick. Fire those up and create a diversion! We must hide.’

Fortuny hesitated.

‘Do it for your friend. I will explain later,’ said Holmes, and he dashed to the cupboard door and opened it, but it was filled to bursting with glass retorts, tubing and other equipment. No room to hide!