She had not seen the pictures, and they had not been mentioned.

She ought to have thought of some recent news that came to Paris, for we were but seven months from there and had been living there a couple of years when we started on this trip; but instead of that she thought of an incident of our brief sojourn in Paris of sixteen years before.
Here was a clear case of mental telegraphy; of mind-transference; of my mind telegraphing a thought into hers.

How do I know?
Because I telegraphed an error.

For it turned out that the pictures did not represent the killing of Lulu at all, nor anything connected with Lulu.
She had to get the error from my head--it existed nowhere else..