246 Intelligence and Miscellaneous Articles, 



Iodine vapour disperses light in a different direction to any sub- 

 stance yet studied ; that is, a prism full of iodine vapour refracts red 

 rays to a greater extent than blue rays. 



In the first communication I made on this subject, I said that, by 

 filling with iodine vapour the prism of my apparatus, " the image of 

 a brilliantly illuminated slit appeared composed of two distinct juxta- 

 posed parts, one red, the other blue." I had remarked that the order 

 of the two colours was not the same as in the spectra produced by 

 all the substances previously studied. Yet I would not announce 

 this result, so contrary to what is usually seen, until I had studied it 

 in all its details, and especially after having arranged my apparatus 

 so as to enable other persons to witness the phenomenon. 



After several attempts, I had first to get prisms of porcelain made ; 

 for metal prisms, whether gilt or enamelled, did not resist the action 

 of iodine vapour. I had then to combine my system of illumination, 

 so as to overcome by a sufficient luminous intensity the opacity of 

 iodine vapour. To put my experiments beyond a doubt, I had then 

 to study the appearances observed when a luminous image is viewed 

 by a small number of simple lights. I had thus occasion to put in 

 evidence the want of achromatism of the eye, which led to a great 

 number of consequences. This forms the subject of a research which 

 I communicated to the Academy about six weeks ago, and which now 

 appears in the Annates de Chimie et de Physique. 



As confirmation of the fact which I advance, the following are 

 the various proofs to which I have submitted it : — 



1 . The effect observed does not depend on a special or accidental 

 arrangement of the apparatus, or of the glasses which compose the 

 prism, since different prisms formed by different glasses always give 

 the same result. 



2. A glass prism which gives a deviation in the same direction as 

 the prism of iodine vapour, and almost equal to it (about 11'), 

 virtually achromatizes the image ; the dispersion of iodine vapour is 

 thus opposite that of glass. 



3. In successively illuminating the slit of my collimator by the red 

 and bluish violet arising from the dispersion of a solar beam by a 

 flint-glass prism, the red and the blue image are produced at differ- 

 ent parts. An experiment of the same kind may be made by inter- 

 posing different coloured-glasses. This shows that the refrangibility 

 of the red ray is really greater than that of the blue ray in the iodine 

 vapour; hence the phenomenon cannot be explained by a transforma- 

 tion of radiations, since the light which is red on entering the prism is 

 red on emergence, and so on. 



4. The prism being placed in air, the effect is complicated by the 

 dispersion produced by this medium ; but a direct experiment showed 

 me that the dispersion produced by the air under the same circum- 

 stances was only a few seconds, while that of iodine is about 30". 



The dispersive power of iodine varies inversely as the tempe- 

 rature. 



Besides red and blue, iodine allows ultra-violet rays to pass ; hence 

 the precision of the phenomenon is increased by purifying, by means 



