Pickering and his two men backed up in alarm but held open the door. As Madame drew closer to them, she gently passed her hands near the largest of the tubes. As she did so, the glowing substance inside danced strangely in response! Off to one side, but still in near darkness, Fortuny worked some gas valves then retreated behind us again.

‘Come, spirits, come!’ The red, then the blue, then the green—all glowing substances—snaked and writhed with the touch of her fingers.

It was indeed unearthly, weird and beautiful – truly magical, as if Madame Borelli were a real-life sorceress. For a moment, I forgot where we were and what we were doing.

‘Good God!’ exclaimed Pickering. His two constables backed into the hall.

‘Sir, can we go, please?’ begged one.

‘Ha ha, yes!’ whispered Holmes.

Just on the other side of the cupboard door, I heard Leo Vitale’s voice. ‘Oh, Cosimo, I despair!’

‘But isn’t it magnificent?’ returned Fortuny.

‘Who’s there?’ shouted Pickering. ‘Holmes? Vitale?’ He turned to his men. ‘Find the lights and turn them on!’

But his men hesitated. Neither dared enter the room.

Cosimo Fortuny stepped forward through the murk and into our view. The glowing lights cast coloured patterns on his face.

‘It is I, Dr Fortuny. You interrupt my experiments. What do you want?’

‘We are looking for Leo Vitale,’ boomed the sergeant, squinting into the dark. ‘This is his laboratory, isn’t it? What is all this … this fairyland nonsense? Turn it off and light the lights. The normal lights.’

‘This is Mr J.J. Thomson’s laboratory. It is also my laboratory, and Leo Vitale is sometimes here,’ said Fortuny.

‘I am here now,’ cried Leo in a shaking voice. ‘I am here, and I am innocent.’

‘The young fool!’ Holmes hissed.

At this point, one of the policemen found a master switch. The electrified illumination came on brightly, and the glow of the tubes seemed suddenly pale and weak.

Vitale stepped forward, passing into our line of sight. ‘I am the one you want,’ he said. ‘I am innocent, and I will prove it.’ I wondered if the boy, in his frenzy of valour, would expose us as well.

Pickering grinned. ‘Leo Vitale, I am arresting you for the murder of Miss Odelia Wyndham. You were heard arguing with her at two in the morning last night and she died not long after that. Come forward and make no sudden moves. Extend your hands before you.’

Fortuny gasped. ‘What the devil?’

Vitale reached Pickering and held his hands out. Pickering snapped the cuffs on him roughly. ‘Take him in to the station,’ he said.

‘I had nothing to do with her death, sir,’ said Vitale as he was hurried out the door. Pickering paused, scanning the room.

‘Good day, Officer,’ said Fortuny dismissively.