212 Prof. Potter on Physical Optics. 



a reddish yellow sediment subsides, which seems to be 

 the peroxide of iron. The decomposition of the cyanide 

 takes place much more rapidly when strips of filtering paper 

 or linen are immersed in a solution of the salt and exposed 

 to the action of solar light. In a very short time that part 

 of the strip turned towards the sun becomes yellow, whilst 

 the opposite side remains colourless, or nearly so. If strips 

 of paper moistened with the solution of the common prussiate 

 of potash are closed up in»glass bottles containing air, they also 

 turn yellow by exposure to the sun, and a strong smell of 

 prussic acid is perceptible in the vessels after a short time. In 

 the shade no such action takes place. A large piece of linen 

 cloth drenched with a solution of the yellow salt, after having 

 been exposed in the open air to the action of solar light for 

 thirty-six hours, had turned deeply yellow, and yielded, when 

 treated with distilled water, a deep yellow solution, which on 

 being filtered and heated to boiling became turbid, and depo- 

 sited flakes of peroxide of iron. The same solution exhibited 

 a stronger alkaline reaction than the solution of the common 

 prussiate does. From the facts stated, it appears that the 

 yellow ferro-cyanide is decomposed by light into prussic acid, 

 oxide of iron and potash, and a compound formed yielding 

 with water a yellow solution. Is that compound carbonate 

 of potash and peroxide of iron ; and do the constituent gases 

 of the atmosphere take part in the decomposition besides the 

 solar light ? Further experiments must answer those ques- 

 tions. A limpid solution of the red cyanide also becomes 

 turbid when exposed to the action of solar light, prussic acid 

 being evolved and peroxide of iron thrown down. 



XL. A Reference to former Contributions to the Philoso- 

 phical Magazine, on Physical Optics. By Prof. Potter, 

 A.M., F.C.P.S., late Fellow of Queen's College, Cambridge, 

 $c* 



"l^TITHOUT the slightest wish to interfere in the contro- 

 ™ ™ versies of others, I now beg to refer the readers of the 

 Philosophical Magazine to my papers in the Magazines for 

 January 1840 and May 1841. In the former, at page 20, I 

 have shown Mr. Green's formula for the intensity of reflected 

 light to fail entirely as a representation of nature ; and in the 

 latter I have shown the peculiar refraction near the optic axes 

 of biaxal crystals not to be represented by Sir William Ha- 

 milton's analytical deductions from Fresnel's equation to the 

 wave surface in biaxal crystals. 



* Communicated by the Author. 



