liO 



May 5, 1851. 



Of the Transformation of Hypotheses in the History of Science. 

 By W. Whewell, D.D. 



The author remarks that new theories supersede old ones, not 

 only by the succession of generations of men, but also by transfor- 

 mations which the previous theories undergo. Thus the Cartesian 

 hypothesis of vortices was modified so that it explained, or was sup- 

 posed to explain, a central force : and then, the Cartesian philoso- 

 phers tried to accommodate this explanation of a central force to the 

 phenomena which the Newtonian principles explained ; so that in 

 the end, their theory professed to do all that the Newtonian one did. 

 The machinery of vortices was, however, a bad contrivance to pro- 

 duce a central force ; and when it was applied to a globe, its defect 

 became glaring. Still however, the doctrine of vortices has in it 

 nothing which is absurd anterior to observation. The " nebular 

 hypothesis" is a hypothesis of vortices with regard to the origin of 

 the system of the universe, and is now held by eminent philosophers. 

 Nor is the doctrine of the universal gravitation of matter at all in- 

 consistent with some mechanical explanation of such a property ; 

 for instance, Le Sage's. We cannot say therefore that if the planets 

 are moved by gravitation, they are not moved by vortices. The Carte- 

 sians held that they were moved by both ; by the one, because by 

 the other. 



Like remarks may be made with respect to the theories of mag- 

 netism and of light. 



May 19, 1851. 



On the Colours of Thick Plates. By G. G. Stokes, M.A., Fellow 

 of Pembroke College, and Lucasian Professor of Mathematics in the 

 University of Cambridge. 



By the expression " colours of thick plates " is usually understood 

 the system of coloured rings, discovered by Newton, which are formed 

 on a screen when the sun's light is transmitted through a small hole 

 in the screen, and received perpendicularly upon a concave mirror of 

 quicksilvered glass, placed at such a distance from the screen that 

 the image of the hole is at the same distance from the mirror as the 

 hole itself. The brilliancy of the rings, as was afterwards discovered, 

 is greatly increased by tarnishing the surface of the mirror; and it 

 is also advantageous to use a lens to collect the sun's rays, and to 

 place the screen so that the small hole may be situated at the focus 

 of the lens. These rings were first explained on the undulatory 

 theory by Dr. Young, who attributed them to the interference of 

 two streams of light, of which the first is scattered at the tarnished 

 surface of the mirror, and then regularly reflected and refracted, while 

 the second is regularly refracted and reflected, and then scattered in 

 coming out of the glass. The theory has been worked out in detail 



