How dense you are, Jack!" put in Hart.
Now, this was the first Robinson had heard of whiskers in connection with the crime.

He remembered Elkin's make-up as Svengali, of course, and could have kicked himself for not associating earlier a set of sable whiskers with the black wig and the bullet-torn hat.
But, Owd Ben! What figure did that redoubtable ghost cut in the mystery?
"There are certain _lacunae_ in your otherwise vigorous and thrilling story, constable," went on Hart.
"Very likely, sir," agreed Robinson, much to the surprise of his hearers.

He had not the slightest notion what a _lacuna_, or its plural, signified.

He was only adopting Furneaux's advice, and trying to be civil.
"Ah, you see that, do you ?" said Hart.

"Well, fill 'em in.