A study in scarlet, eh? Why shouldn't we use a little art jargon? There's the scarlet thread of murder running through the colourless skein of life, and our duty is to unravel it, and isolate it, and expose every inch of it.

It is of the highest importance in the art of detection to be able to recognise out of a number of facts which are incidental and which are vital.

The soul is swayed by the waters.

I say, Watson,’ he whispered, ‘would you be afraid to sleep in the same room as a lunatic, a man with softening of the brain, an idiot whose mind has lost its grip?’ ‘Not in the least,’ I answered in astonishment. ‘Ah, that’s lucky,’ he said, and not another word would he utter that night.

Amberley excelled at chess - a mark, Watson, of a scheming mind.

Circumstantial evidence is occasionally very convincing, as when you find a trout in the milk, to quote Thoreau's example.

...it was a huge creature, luminous, ghastly, and spectral. I have cross-examined these men, one of them a hard-headed countryman, one a farrier, and one a moorland farmer, who all tell the same story of this dreadful apparition, exactly corresponding to the hell-hound of the legend. I assure you that there is a reign of terror in the district, and that it is a hardy man who will cross the moor at night.