IN a letter to NATURE1 I observed that the culture of diatoms in polarised light would be an interesting piece of research. Later, I placed a culture in a small tube in the path of light from a Nicol prism, and a similar tube using the same culture medium, in ordinary daylight: at night, light from the sky was admitted as in the day. The rate and prolificacy of reproduction were approximately the same. It appears from this that it is the length of the period of exposure, and the intensity of the light which are the chief factors; the polarisation of the light is of little importance. In this case it looks as though reproduction at night has no connexion with lunar periodicity.