When I first saw Le Roux's account of his very singular discovery of the abnormal refraction of iodine vapour, I was inclined to attribute the phenomenon to something similar to over-correction of an achromatic combination. In fact, if a hollow prism be filled with a mixture of two gases or vapours, one of which is more refractive than air, the other less refractive; while the second body is more dispersive than the first; it is easy to see that Le Roux's result might be obtained, although each of the substances employed is free from anomalous refractive properties. In a recent conversation with Mr Talbot, I happened to mention the subject, and I learned from him his remarkable observation just laid before the Society.