'Certainly I can,' said Mr. Cupples, peering at it with great interest. 'It is an ordinary glass bowl. It looks like a finger-bowl. I see nothing odd about it,' he added after some moments of close scrutiny.
'I can't see much myself,' replied Trent, 'and that is exactly where the fun comes in. Now take this little fat bottle, Cupples, and pull out the cork. Do you recognize that powder inside it? You have swallowed pounds of it in your time, I expect. They give it to babies. Grey powder is its ordinary name–mercury and chalk. It is great stuff. Now, while I hold the basin sideways over this sheet of paper, I want you to pour a little powder out of the bottle over this part of the bowl–just here. . . . Perfect! Sir Edward Henry himself could not have handled the powder better. You have done this before, Cupples, I can see. You are an old hand.'
'I really am not,' said Mr. Cupples seriously, as Trent returned the fallen powder to the bottle. 'I assure you it is all a complete mystery to me. What did I do then?'
'I brush the powdered part of the bowl lightly with this camel-hair brush. Now look
