the Cause of coloured concentric "Rings, 87 



second, and almost as bright as the first: I have easily 

 counted 7, 8, and 9 rings. 



XT. Of the Effect of Pressure on the Colour of the Rings, 



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

 is laid on a plain ?lip of glass, the first colours that make 

 their faintest appearance will be red surrounded by green ; 

 the smallest pressure will turn the centre into green sur- 

 rounded by red : an additional pressure will give a red centre 

 again, and so on till there have been so many successive al- 

 terations 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 hs 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 phaenomena, will not be necessary. 



When 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, grfcen, yellow, and black, 

 at pleasure. In this case 1 used an eye-glass of one inch 

 focus ; but as it requires much practice to manage such 

 small glasses, the experiment may be more coi^vcniently 

 made by placing a double convex lens of two-inch focus on 

 a plain slip of glass, and viewing the rings by an eye-glass of 

 S-J- inches ; then having first brought the lens into full con- 

 tact, the rings will be only black and white, but by gently 

 Jfting up or tilting the lens, the centre of the rings wilL as- 

 suipe various colours at pleasure. 



F4 XII. Of 



