"deflect the bark"?

Is this to say: where they detract from the right approach or somehow undermine the good effort?

For Myers and his colleagues, the "very raison d'etre [of psychical research] is the extension of scientific method, of intellectual virtues . . . .into regions where many a current of old tradition, of heated emotion, even of pseudo-scientific prejudice, deflects the bark"

Solution 1:

Deflect has a number of usages. The given usage, deflects the bark is a metaphor for deflecting criticism.

If you deflect something such as criticism or attention, you act in a way that prevents it from being directed towards you or affecting you.

Here, a bark would be the response of a dog, perhaps lacking intellect or insight; or perhaps in anger. It could be either off-the-cuff or part of a larger context, as @EdwinAshworth suggests.