252 Professor Powell on the Repulsion produced [April 



between lenses, and applying heat, which would afford a 

 simple mode of deciding the question, if there be any 

 separation of the glasses by repulsion, since it would be 

 rendered visible by the contraction of the rings. As to the 

 error which might arise from the warping of the upper 

 glass by the heat, it will be evident, on a little considera- 

 tion, that heat applied outside of either glass will tend, by 

 the change of figure, in every case, in the first instance, to 

 diminish the angle of contact : that is, if no other cause 

 interfere, to make the rings enlarge without altering the 

 central tint, until the curvature become equal to that of the 

 convex surface. 



I invariably found, however, that from the first moment 

 the rings regularly contract, and the central tint descends in 

 the scale till the whole vanishes. There are, however, several 

 precautions necessary to be attended to. If the glasses be 

 more than very slightly convex, the portion of surface 

 throughout, which they approach sufficiently near for the 

 repulsion to act, is very small. This may render the total 

 effect far too weak to overcome the weight of the upper 

 glass, or even its inertia, though placed vertically. With 

 surfaces of such curvature as to give the first bright ring a 

 diameter of about 0*3 inch, on placing a red hot poker a 

 little above the glasses the effect never failed to be produced. 

 Upon the whole, the experiments, though simple in principle, 

 certainly require some care ; but with all precautions, and 

 after the most careful consideration of all causes which can 

 have tended to produce or affect the result, it appears to 

 me that the separation of the glasses through the extremely 

 small, but finite and known spaces, whose changes are 

 indicated by the degradation of the tints, can only be due 

 to the real action of a repulsive power, produced or excited 

 between the surfaces of the glasses by the action of heat. 



There are many questions relating to the nature and 

 properties of this repulsive power, which are immediately 

 suggested, and some of which appear capable of solution by 

 variations of the same method. 



The distance at which the repulsive power can act is 

 shown, by these experiments, to extend beyond that at 

 which the most extreme visible order of Newton's tints is 

 formed. But I have also repeated the experiment success- 

 fully with the colours formed under the base of a prism 

 placed upon a lens of very small convexity ; and according 



