XX. The action of light on bacteria.—III

s and Blunt, the first who observed the destructive action of light on a, had already raised the question as to which of the various rays of the spectrum are active, and they made some experiments with coloured screens. The objection to all their results is that they did not use pure cultures. Arloing, Janowsky, Geisler, and Kotljar employed pure tube cultures—broth, gelatine, potato and exposed them behind various screens, endeavouring to judge of the of the light by the relative rapidity with which the broth becomes turbid, or which the masses of colonies spread over the surface of the solid media, &c. The chief objection to these methods is that it is very difficult to compare and it two separate culture-tubes of this kind, though it should be noticed that Janowsky employed a very ingenious expedient for ensuring at least that his broth- should be exactly similar to start with.