Mr. R. W. Fox on the Laws of Magnetic Attraction, 4S9 



the solution exercises no general absorptive action. The 

 sharp and narrow black band which is thus formed constitutes 

 a fixed 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 substances, which in most cases 

 are unattainable. 



In order to render this line or band of real use in practical 

 optics, I have endeavoured to fix its place with as great ac- 

 curacy 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«; 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 1-330701, 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 solution 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 transparency, through all shades 

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

 shades of pale yellow, pale orange, red, and blue, in candle- 

 light. 



Belleville, by Kingussie, March 21, 1835. 



LIV. Some Remarks on the Laws of Magnetic Attraction. 

 By R. W. Fox * 

 [" HAVE shown in a paper which has been published in the 

 -*• Lond. and Edinb. Philosophical Magazine, Third Series, 

 vol. v. p. 1. that the reciprocal force of two magnets, when 

 placed within certain very small distances from each other, 

 will vary in the simple inverse ratio of the distance; and that 

 the magnetic elements, or at least pulverized loadstone, or 

 other ferruginous particles adhering to the poles of the mag- 

 nets, will, under such circumstances, become arranged in 

 parallel straight lines; thus explaining, as I conceived, the 

 cause of the existence of this law, — such parallel lines being, 

 as it appeared to me, incompatible with the law of the in- 

 verse ratio of the square of the distance. 



* Communicated by the Author. 



