Prof. Stokes on the Internal Distribution of Matter, §*c. 235 



slit ; and that friction is so reduced that it is not even capable of pre- 

 venting the lenses sliding by the force of their own gravity as soon 

 as the whole tube is placed in the least degree in an inclined position. 

 This is very advantageous ; for it supplies the means (about to be 

 explained) of rendering the whole instrument complete and perfect, 

 making it self-acting with any velocity required, and in its regular 

 action independent of any error in the appreciation of time during 

 the exposure. All these advantages it might have been at first con- 

 sidered impossible to obtain ; at the same time considerable difficul- 

 ties are avoided in the working out of the plan of a moving focus 

 which might have constituted a decided impediment to its adoption. 

 But now that the movement can be imparted to the whole with the 

 least power, we are able to communicate it with an ordinary clock 

 work, and by means of a regulator to produce the separation of the 

 two lenses required in any given time according to the exposure judged 

 necessary beforehand, on account of the greater or less intensity of 

 light. 



Thus are effectually removed all the practical objections which at 

 first were offered against the adoption of this great improvement in 

 photography — as in the sequel it is sure to be found, although, like 

 many other most useful and new inventions, it has been exposed to 

 the opposition and sneers of those who, mainly influenced by preju- 

 dices, are always prompt to judge without taking the trouble of exa- 

 mining and understanding the conscientious labours which may have 

 been performed at a great expense of money, time, and thought, and 

 in the present case without any possible view of pecuniary remunera- 

 tion, but only in the love of progress and for the honour of contri- 

 buting to the advancement of the art. 



May 16. — William Bowman, Esq., V.P., in the Chair. 



The following communications were read : — 



"On the Internal Distribution of Matter which shall produce a 

 given Potential at the Surface of a Gravitating Mass." By G. G. 

 Stokes, M.A., Sec. R.S. 



It is known that if either the potential of the attraction of a 

 mass attracting according to the law of the inverse square of the 

 distance, or the normal component of the attraction, be given all 

 over the surface of the mass, or any surface enclosing it (which 

 latter case may be included in the former by regarding the internal 

 density as null between the assumed enclosing surface and the actual 

 surface), the potential, and consequently the attraction, at all points 

 external to the surface and at the surface itself is determinate. This 

 proposition leads to results of particular interest when applied to the 

 Earth, as I showed in two papers published in 1849*, where among 

 other things I proved that if the surface be assumed to be, in 

 accordance with observation, of the form of an ellipsoid of revolution, 



* "On Attractions, and on Clairaut's Theorem," Cambridge and Dublin 

 Mathematical Journal, vol. iv. p. 194 ; and " On the Variation of Gravity at the 

 Surface of the Earth," Cambridge Philosophical Transactions, vol. viii. p. 672. 



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