As regular readers of this blog will know, I have four dogs. These are my pets, my friends, even, very possibly, my substitute children. But they are most definitely not - NOT - my familiars.
Witch hunters years ago got it into their head that witches kept animals who drank their blood - their 'familiars,' and these were often owls and cats.
But although I have yet to meet a witch who really does keep a 'familiar' there could be a good reason why cats and owls were so popular. These are both creatures that hunt by night, and can operate in almost total darkness. Owls can detect a mouse some distance away, but not a piece of dead meat even if it's the same size and shape.
Basically what they are doing is seeing in infrared, and this is another way of seeing the 'aura.' Some people can do this, perhaps because their vision is better at the infrared end of the spectrum. This has something to do with the cone shaped cells in the retina, which just cannot cope with these visual wavelengths.
A traditional way of seeing the aura has been to look at someone in dim light, with your eyes partly closed and the light striking the corner of the eye. Personally I'm not sure about this. Some people swear by the method, others don't find it makes a blind bit of difference.
Mr Penry can see the aura perfectly well in broad daylight and never half closes his eyes (unless savouring a particularly good malt whiskey). 
