OF THE OXALATE OF CHROMIUM AND POTASH. 93 



side of the lines D, E, are wholly destroyed. In this state the prism gives two distinct 

 images of objects, viz. a red and a greenish blue image, which are considerably sepa- 

 rated. As the absorption advances, the green on the blue side of E, and the blue on 

 the violet side of F, gradually disappear, till a pure blue image about F alone remains, 

 and this too wholly vanishes by an increased thickness of the solution, leaving the red 

 rays unabsorbed. 



While these changes are going on throughout the spectrum, a specific action is 

 exerted upon a red ray between A and B of Fraunhofer, and in that very part of the 

 spectrum over which the solution exercises no general absorptive action. The sharp 

 and narrow black band which is thus formed constitutes a Jixed line in all artificial 

 lights, and also in solar and day light, which will enable philosophers to measure the 

 refractive powers of all bodies in reference to this line with an accuracy which could 

 not otherwise be obtained, unless by the use of fine prisms of the refracting sub- 

 stances, which in most cases are unattainable. 



In order to render this line or band of real use in practical optics, I have endea- 

 voured to fix its place with as great accuracy as possible. Between the lines A, B of 

 Fraunhofer there is a group of lines nearly bisecting the space A B, which he has 

 marked a in his map. The dark band lies in the space B a ; and if we designate it 

 by the letter X, its position is such that B X = J B a, or the index of refraction in the 

 Water spectrum, of the rays which are absorbed at the band X is almost exactly 

 r330701, the temperature of the water being 65° of Fahrenheit. 



The relations of this salt to common and polarized light may be readily examined 

 and finely exhibited by placing upon a plate of glass a few drops of a saturated solu- 

 tion of it in water. If the crystals are slowly formed they will be found of various 

 thicknesses, each thickness exhibiting a different colour, varying from perfect trans- 

 parency, through all shades of pale yellow, green, and blue, in daylight, and through 

 all shades oi pale yellow, pale orange, red, and blue, in candlelight. 



Belleville, by Kingussie, 

 March 2\st, 1835. 



