HERSCIIEL ON COLOURED RINUS, \$$ 



XI. Of the Effect of Pressure on the Colour of the Rings. 



When a double convex object glass of 14 or 15 feet focus Their colours 



is laid on a plain slip of glass, the first colours that make their affccted b ^ 



r . pressure* 



faintest appearance will be red surrounded by green; the 



smallest pressure will turn the centre into green surrounded 



by red : an additional pressure will give a red centre again, 



and so on till there have been so many successive alterations, 



as to give us six or seven times a red centre, after which the 



greatest pressure will only produce a very large black one 



surrounded by white. 



When the rings are seen by transmission, the colours are 

 In the same manner subject to a gradual alternate change 

 occasioned by pressure , but when that is carried to its full 

 extent, the centre of the rings will be a large white spot sur- 

 rounded by black. 



The succession and addition of the other prismatic colours, 

 after the first or second change, in both cases is extremely 

 beautiful ; but as the experiment may be so easily made, a 

 description, which certainly would fall short of an actual view 

 of these phenomena, will not be necessary. 



W r hen the rings are produced by curves of a very short 

 radius, and the incumbent lens is in full contact with the slip 

 of glass, they will be alternately black and white; but by 

 lessening the contact, I have seen, even with a double convex 

 lens of no more than two tenths of an inch focus, the centre 

 of the rings white, red, green, yellow, and black, at pleasure. - 

 In this case I used an eye-glass of one inch focus; but as it 

 requires much practice to manage such small glasses, the 

 experiment may be more conveniently made by placing a 

 double convex lens of 2 inches focus on a plain slip of £>lass, 

 and viewing the rings by an eye-glass of 2f inches; then 

 having first brought the lens into full contact, the rings will 

 be only black and white, but by gently lifting up or tilting 

 the lens, the centre of the rings will assume various colours 

 at pleasure. 



XII. Of diluting and concentrating the Colours. 



Lifting up or tilting a lens being subject to great uncer- Method of dfc 

 tainty, a surer way of acting upon the colours of the rings is lutin S or con " 



by 



