Minute Mysteries [Detectograms]

Professor Fordney was hunting in the Rockies when informed of a tragedy at one of the camps. Thinking he might be of some help, he went over, and, after introducing himself, Butler, the victim's companion, told him of the accident.

'When Marshall hadn't returned to camp at nine o'clock last night, I was a bit worried because he didn't know these mountains. There wasn't a star out and it was dark and moonless, so I decided to look around for him. We're five miles from anyone, you know.

'Putting more wood on the fire, I set out. After searching for an hour, I was coming up the slope of a ravine when I saw a pair of eyes shining out at me in the dark.

'Calling twice, and getting no answer, I fired, thinking it was a mountain lion. Imagine my horror when I reached the spot, struck a match, and saw I had nearly blown the head off Marshall. A terrible experience!

'I carried his body back to camp and then walked to the nearest house to report the accident.'

'How far from camp did you find Marshall?' asked Fordney.

'About a quarter of a mile.'

'I see your right hand is bandaged. How do you manage to shoot with it?'

'Oh, I use either hand.'

'Mind if I look at the gun?'

'Not at all,' said Butler, handing it over.

'H'm, European make, I see. Had it long?'

'No, it's rather new.'

'Why did you deliberately murder Marshall?' demanded Fordney abruptly... 'for that's what you did.'

How did the Professor know Butler had murdered his companion?