304 Royal Society. 



-£+T(l-3 sin*\), whereT=^(A—B): the attraction of the sphe- 

 roid on P perpendicular to PO and urging P towards the equator is 

 also easily shown to become T sin 2\. 



Now that the point P may be at rest, it is necessary that the tan- 

 gential component of the central force acting along PO should be 

 equal to the sum of the tangential components of the centrifugal 

 force (acting on P perpendicular to the earth's axis), and of the force 

 perpendicular to PO ; this condition gives an equation from which 

 Clairaut's theorem follows instanter, due regard being had to the 

 difference of the polar and equatorial gravities as determined by the 



general expression ii+T(l — 3 sin 9 \), and the ellipticity of the ex- 

 terior surface being supposed so small that its square and higher 

 powers may be rejected. 



" On the Change of Refrangibility of Light."— No. II. By 

 Professor Stokes, M.A., F.R.S. 



The principal object of this paper is to explain a mode of obser- 

 vation by means of which the author found that he could exhibit, 

 with ordinary day-light, the change of refrangibility produced by 

 substances opake as well as transparent, even when they possessed 

 only a low degree of sensibility. The method requires hardly any 

 apparatus ; it is extremely easy in execution ; and it has the great 

 advantage of rendering the observer independent of sun-light. On 

 these accounts the author conceives that it might be immediately 

 applied by chemists to the discrimination between different sub- 

 stances. The method is as follows : — 



A large hole, which ought to be several inches in diameter, cut in 

 the window-shutter of a darkened room, serves to introduce the light, 

 and a small shelf, blackened on the top, attached to the shutter im- 

 mediately underneath the hole, serves to support the objects to be 

 examined, as well as one or two absorbing media. The hole is 

 covered by an absorbing medium, called by the author the principal 

 absorbent, which is so selected as to let through, as far as may be, 

 the feebly illuminating rays of high refrangibility, as well as the in- 

 visible rays still more refrangible, but to stop the rays belonging to 

 the greater part of the visible spectrum. A second medium, called 

 by the author the complementary absorbent, is chosen so as to be as 

 far as possible transparent with regard to those rays which the first 

 medium stops, and opake with regard to those which it lets through. 

 The object to be examined is placed on the shelf, and viewed through 

 the second medium. If the media be well-selected, they together 

 produce a very fair approach towards perfect darkness ; and if the 

 object appears unduly luminous, that arises in all probability from 

 " fluorescence." To determine whether the illumination be really 

 due to that cause, it is commonly sufficient to remove the comple- 

 mentary absorbent from before the eyes to the front of the hole, 

 when the illumination, if it be really due to fluorescence, almost 

 wholly disappears ; whereas, if it be due merely to scattered light 



