suspended above the three rows of benches which ran the length of the room.
Mercurial lamps do not give a green light, as he knew, but a violet light, and the green effect was produced by shades of something which Beale thought was yellow silk, but which he afterwards discovered was tinted mica.
At intervals along the benches sat white-clad figures, their faces hidden behind rubber masks, their hands covered with gloves. In front of each man was a small microscope under a glass shade, a pair of balances and a rack filled with shallow porcelain trays. Evidently the work on which they were engaged did not endanger their eyesight, for the eye-pieces in the masks were innocent of protective covering, a circumstance which added to the hideous animal-like appearance of the men. They all looked alike in their uniform garb, but one figure alone Beale recognized. There was no mistaking the stumpy form and the big head of the Herr Professor, whose appearance in Oliva Cresswell's room had so terrified that young lady.
He had expected to see him, for he knew that this old German, poverty-stricken and ill-favoured, had been roped in by van Heerden, and Beale, who pitied the old man, had been engaged for a fortnight in trying to worm from the ex-professor of chemistry at the University of Heidelberg the location of van Heerden's secret laboratory. His efforts had been unsuccessful. There was a streak of loyalty in the old man, which had excited an irritable admiration in the detective but had produced nothing more.
Beale's eyes followed the benches and took in every detail. Some of the men were evidently engaged in tests, and remained all the time with their eyes glued to their microscopes. Others were looking into their porcelain trays and stirring the contents with glass rods, now and again transferring something to a glass slide which was placed on the microscope and earnestly examined.
Beale was conscious of a faint musty odour permeating the air, an indescribable earthy smell with a tang to it which made the delicate membrane of the nostrils smart and ache. He tied his handkerchief over his nose and
