between two Substances of different refractive Powers, 25 



bubbles are rubbed out, Newton's rings are seen, very large 

 though slightly irregular, with the black spot in the centre. 

 The rings in question are seen through this black spot, and 

 consequently are not injured by the effects of reflexion. The 

 water seems to have the power of bringing the lens and prism 

 into closer contact than is otherwise attainable*: for I am 

 well convinced that no force that could be applied without in- 

 juring them would bring them so near together as to exhibit 

 the central black. 



For the denser medium I have used a diamond with a sur- 

 face of about T ] ? inch in diameter, mounted in a ring: for the 

 use of which 1 am indebted to the politeness of William John 

 Broderip, Esq. Vice-President of the Geological Society. 

 When the lens and prism were placed on this, a small system 

 of rings was seen perfectly distinct and well formed, the dia- 

 meter of the fifth ring not exceeding h of the diameter of the 

 surface. 



These rings were examined with the combination of tour- 

 maline and doubly-refracting prism that I have described. 

 When the angle of incidence was small, the rings formed by 

 light polarized perpendicular to the plane of reflexion were 

 seen sufficiently vivid, with black centre, accompanied by the 

 other set of rings which were faint. When the angle of inci- 

 dence reached the polarizing angle of the glass, the first set of 

 rings disappeared. On increasing the angle, the first set of 

 rings was again seen with centre white. In the most favour- 

 able state, the first set of rings was much more faint than the 

 second, but not so faint that there could be the slightest doubt 

 upon the fact of the existence of the rings and the whiteness 

 of the centre, as I saw them repeatedly with every change in 

 the arrangement of the apparatus, and saw a succession of 

 several rings. The white spot appeared larger than the dark 

 spot in the other set of rings, but this I imagine is owing 

 merely to the undefined nature of the spots, and to the circum- 

 stance that, in appreciating their comparative extent, the eye 

 always gives credit to the brightness for a greater surface than 

 it can properly claim. In respect of dimensions of correspond- 

 ing parts, I could see no difference. On increasing the angle 



* I may here mention a curious circumstance which occurred to me in 

 the use of this combinaiion. After leaving the prism, with the lens hanging 

 to its lower surface, for one or two days, the water contracted itself to a 

 spot (having partly gone off, I suppose, by evaporation) of about |- inch 

 in diameter, its outline following most accurately the course of one of the 

 rings (I think the third) even in its deviations from symmetry. In this state 

 I was not able to move the lens upon the prism, though I applied a force 

 parallel to the surface of the prism sufficiently great to shiver large splinters 

 from the lens. On dipping them into water they instantly dropped asunder. 

 Third Series. Vol. 2. No. 7. Jan. 1833. E 



