288 Sir D. Brewster on certain Changes of Colour 



point whose coordinates in that plane are xandj/, is ascer- 

 tained to come under the formula 



C 

 1+A* + By 

 A, B and C being constants, dependent upon the number and 

 positions of the points of support, and upon the forces impressed 

 upon the system. 



From this ibrmula it follows, that there is a certain line, 



distant from the origin by a quantity equal to — — _ , and 



v A" -f- -t> 



inclined to the axis of x at an angle whose tangent is — tt ; 



about which the moments of all the pressures are the same. This 

 line may be properly called the axis of least pressure, as the 

 principle whence its existence and properties have been de- 

 duced, may be designated the -principle of least pressure. 



Where the points of support are in the same line, this axis 

 resolves itself into a point ; and it follows that the moments of 

 the resistances about a certain point in the line in which they 

 act, are all equal ; — a result which is verified by the known 

 conditions of the pressure upon two points of support. 



LI. Notice respecting certain Changes of Colour in the Choroid 

 Coat of the Eyes of Animals. By Sir David Brewster, 

 LL.D. F.R.S. 



IN the Number of this Journal for August last, (p. 87) Mr. 

 Fielding has published some interesting experiments respect- 

 ing certain changes of colour induced by chemical and other 

 agents on the membrane lining the choroid coat of the eye; and 

 he has particularly described an experiment which shows that 

 when the colours have disappeared by drying, they may be 

 revived by simple immersion in water. 



In the chapter on the colours of natural bodies in the arti- 

 cle Optics in the Edinburgh Encyclopaedia, I have brought 

 forward this fact as illustrating and supporting Sir Isaac New- 

 ton's Theory of the Colours of Natural Bodies, and I am in- 

 duced to mention this at present, not only for the purpose of 

 giving to Dr. Drummond of Belfast, the merit of having first 

 made this curious experiment, but also of making some further 

 observations upon it in reference to its connexion with the 

 theory of Newton. The following is the passage in which it 

 is mentioned. 



" Dr. Drummond of Belfast observed, that the membrane 



