Dr. Hudson on the Intensity of Light fyc 



359 



of the circuit produced only a shrinking of 47 divisions. There 

 was a tendency on the part of the bar or of the mirror to per- 

 sist in the condition superinduced by the magnetism. The 

 passing of a cab in this instance caused the scale to move from 

 517 to 534; that is, it made the shriuking 64 instead of 47. 

 Tapping the bar produced the same effect. 



"The bar employed here was a wrought-iron square core, 1*2 

 inch a side and 2 feet long. 



" The following Tables will sufficiently illustrate the perform- 

 ance of the instrument in its present condition. In each case 

 are given the figures observed before closing, after closing, and 

 after interrupting the circuit. Attached to each Table also are 

 the lengthening produced by magnetizing and the shortening 

 consequent on the interruption of the circuit : — 



Circuit. 



Scale 

 10 cells. 





Circuit. 



Scale. 

 20 cells. 





Open 



Closed 



Broken ... 



Open 



Closed . . . 

 Broken ... 



Open 



Closed ... 

 Broken ... 



647 

 516 

 581 



637 

 509 

 579 



632 

 491 

 568 



131 elongation. 

 65 return. 



128 elongation. 

 70 return. 



141 elongation. 

 77 return. 



Open 



Closed ... 

 Broken ... 



Open 



Closed ... 

 Broken ... 



Open 



Closed ... 

 Broken ... 



653 

 465 

 579 



638 

 452 

 568 



632 



472 

 561 



188 elongation. 

 114 return. 



186 elongation. 

 116 return. 



160 elongation. 

 89 return. 



" These constitute but a small fraction of the numbers of ex- 

 periments actually made. There are very decided indications 

 that the amount of elongation depends on the molecular condi- 

 tion of the bar. For example, a bar taken from a mass used in 

 the manufacture of a great gun at the Mersey Iron works, suf- 

 fered changes on magnetization and demagnetization consider- 

 ably less than those recorded here. I hope to return to the 

 subject." 



XLIV. On the Intensity of Light fyc. 

 By Henry Hudson, M.B., M.R.I.A. 



To the Editors of the Philosophical Magazine and Journal. 



Glenville, Fermoy, 

 Gentlemen, March 4, 18/3. 



I THINK Mr. Bosanquet could not have looked into the 

 Astronomer RoyaFs excellent little work ' On the Undula- 

 tory Theory of Light' when he wrote (p. 217) that "The ex- 

 planation given by Airy of the doubling of intensity .... can 



