and Observations on Visible Vibration. 427 



may include a range of so much, and produce the same 

 note from any part of the sector ; but the centre of vibra- 

 tion is always the centre of a sector, and the part to which 

 the bow is applied, becomes, therefore, both a centre of 

 vibration and the centre of a sector. It will, therefore, be 

 seen, that, with vessels with lips or handles, the bow may be 

 allowed considerable range, provided, that in producing the 

 lower tones, the handle or lip be included in one of the 

 vibrating arcs, and in producing the upper tones care must 

 be taken that such be not the case. 



132. In glass vessels, or, indeed, in vessels of any 

 material, the extent of a vibrating arc can always be de- 

 termined by a very simple rule : — Let the vessel contain 

 water, and for any given note count the number of fans 

 produced on the surface, and divide 360 by this number, 

 and the quotient will give the extent of each vibrating 

 arc. Thus, from a glass goblet, suppose a very acute note 

 be produced, and 12 fans be seen upon the surface of the 

 water proceeding from equidistant points of its inner cir- 

 cumference ; 360 divided by 12 will give 30°, the extent of 

 each vibrating arc producing a dodeca-nodal note. 



Salisbury, 2&f/i July, 1836. 



Article V. 



On Accidental Colours^ and Coloured Shadows. 

 By Paul Cooper, Esq. 



The following remarks upon the paper of your correspon- 

 dent, Mr. Tomlinson, published in your Numbers for 

 September and October, are made chiefly with a view to 

 promote the discussion of the interesting subjects to which 

 he has directed our attention. The investigation of opinions 

 thus introduced, appears to me, the likeliest method of 

 arriving at truth ; and, as this is the common object, when 

 the discussion is conducted with the good feeling which 

 such an object is calculated to inspire, it must be indifferent 

 to the parties on which side it is found. 



The point at issue seems to be, whether light produces a 

 continued sensation of uniform intensity ; or, whether it 

 gradually declines after the first impression, so as to render 



