416 Professor Airy on the Phenomena 



upon its plane side, the obtuse angle being over the center of 

 the lens. A drop of water was placed between them. Though 

 its refractive index differs sensibly from that of the glass, yet the 

 reflection at the common surface of the prism and lens is almost 

 totally destroyed, for the following reason. The surface of the 

 lens is I suppose very slightly convex, and when the drop of 

 water is interposed, and the air-bubbles are rubbed out, Newton's 

 rings are seen, very large though slightly irregular, with the 

 black spot in the center. The rings in question are seen through 

 this black spot, and consequently are not injured by the effects 

 of reflection. 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 injuring them would bring them so near together as to 

 exhibit the central black. 



For the denser medium I have used a diamond with a surface 

 of about ^ inch in diameter, mounted in a ring: for the use of 

 which I 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 diameter of the fifth ring 

 not exceeding \ of the diameter of the surface. 



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

 this combination. 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, 1 

 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. 



