410 Proceedings of the Royal Society 
Writing entirely from memory, it is possible that I may have 
fallen into some inaccuracies in this brief account, which, if it 
should be the case, I trust the Society will, under the circumstance, 
kindly excuse. 
P.S . — Since the above remarks were written, the first number of 
Poggendorff’s “ Annalen ” for the present year has been received in 
Edinburgh. This contains a long article by Kundt on the subject 
of Christiansen’s experiment. 
He finds that anomalous spectra are given by all the aniline 
colours, and by permanganate of potash. Such spectra turn back 
upon themselves, generally having the green at one extremity, the 
blue being situated between the green and the red. 
Hence this property is possessed by an extensive class of bodies, 
and must form a new and separate branch of optics. He says that 
the phenomenon only occurs when a very strong solution of the 
substance is employed in the form of a liquid prism of 25°. But 
only the thin extreme edge of the prism is available, the thickness 
of the rest rendering it opaque. He failed in the attempt to form 
a solid prism by mixing collodion with the alcoholic solution, but 
this might perhaps be achieved by other means. In the meantime 
a wide field of experiment is open. 
3. Laboratory Notes. By Professor Tait. 
1. On Anomalous Spectra, and on a simple Direct-vision 
Spectroscope. 
When I first saw Le Roux’s account of his very singular dis- 
covery 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 sub- 
stances 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. I have since, when I had an opportunity, 
