474 The Rev. J. H. Pratt on a Proposition 
red, yellow, green, and blue media, which are absolutely inca- 
pable of reflecting or transmitting certain definite rays of the 
same colour with themselves. 
The true cause of the colours of natural bodies may be 
thus stated: When light enters any body, and is either re- 
flected or transmitted to the eye, a certain portion of it, of 
various refrangibilities, is lost within the body; and the co- 
lour of the body, which evidently arises from the loss of part 
of the intromitted light, is that which is composed of all the 
rays which are not lost; or, what is the same thing, the co- 
lour of the body is that which, when combined with that of 
all the rays which are lost, compose the original light. 
Whether the lost rays are reflected, or detained by a specific 
affinity for the material atoms of the body, has not been ri- 
gorously demonstrated. In some cases of opalescence, they 
are either partly or wholly reflected ; but it seems almost cer- 
tain, that in all transparent bodies, and in that great variety 
of substances in which no reflected tints can be seen, the rays 
are detained by absorption*. 
LXXX. On the Proposition that a Function of § and \ can 
be developed in ONLY ONE Series of Laplace’s Coefficients ; 
the Function being supposed not to become infinite between 
the limits 0 and x of and o and 2% of |. By the Rev. 
J.H. Pratt, B.A.t 
pegs important proposition is, in fact, not proved, but 
assumed, by Laplace in the Mécanique Céleste, II. ii. 
§ 12. Professor Airy pointed out this defect, and gave a 
proof of the proposition in the Cambridge Philosophical ‘Trans- 
actions: but this labours under the restriction of supposing 
the number of terms in the series finite. M. Poisson has con- 
sidered this among numerous other important questions in a 
paper in the Connoisance des Tems for 1829, and also in his 
Théorie Mathématique de la Chaleur, chap. viii. But I con- 
fess it appears to me that the proposition is not proved even 
in these places; though by a slight addition to the reasoning 
the objection to the proof may be removed. 
M. Poisson shows that if p = cosé cos @! + sin @ sin 6! cos 
(¥—w’), and also if (1—2e p + a®)-? = 14a P,+a°?P,+4+.... 
+ 6 Pi vesweh then 
1 2 . 
SANG et 3 ip. ‘ 
L(Y) = af 48a Pitot (28+ IP. + oo} 
SF (4, V') sin déldy’. 79) 
* The views on this subject of Sir John Herschel will be found in a paper 
by that philosopher in Lond. and Edinb, Phil. Mag., vol. iii. p. 401.—Ep1r. 
+ Communicated by the Author. 
