VOL. LXXXVI.] PHILOSOPHICAL TKANSACTIONS. 73g 



either be in a lower or in a higher: let it be in a lower, as that of the square 

 root, then the size of the red would be to the size of the violet as the squares of 

 the forces; that is, as 1625625 to \57200Q: a difference evidently too great; 

 and, a fortiori, of the cube or any other root. On the other hand, if the ac- 

 tion were in a higher ratio, as that of the square, then the particles would be as 

 the square roots of the forces, or nearly as 35.70 to 35.39, a difference evidently 

 too small ; for if the size of the red particles were only -, V greater than that of 

 the violet, and the velocity of both were equal, the momentum, and consequently 

 the intensity of the red, could not so much exceed that of the violet as we find 

 it does, and as seems to be proved by the experiment of Buffon, on accidental 

 colours, who found, that after looking at a white object, when he shut his eyes, 

 it first became violet, then blue, or a mixture of blue and the other colours, and 

 last of all red; so in the impression of the white, compounded of the impressions 

 of all the other rays mixed together, the violet was first obliterated or weakest, 

 and the red last or strongest. To this reasoning on the intensity of the particles 

 as owing to their size, I see only 2 objections that can be made. The one is, 

 that the intensity is increased when the rays are thrown into a focus; but we 

 ■ must recollect that the rays in this case are mixed, and their particles so blended 

 as to be increased in size; for the number of separate rays thrown into one place 

 will not increase their intensity sensibly. The other objection is, that passage in 

 Newton, where he says " that the orange and yellow are the most luminous of 

 all the colours, affecting the senses most strongly." Now, besides that this is 

 an assertion opposed by the positive experiment just now quoted, I think an an- 

 swer may be thus made to it; the whole light, from which the spectrum is never 

 free, which inclines to yellow, and which is composed also of red, abounds in 

 the yellow and orange of the spectrum ; so that both of these colours derive 

 their superior lustre rather than intensity from this circumstance; or if they have 

 any degree of the latter more than the red, it is in fact owing to their mixture 

 with the red and the other rays, which are all in the white. 



Having endeavoured to unfold the property of flexibility, as varied in inflec- 

 tion, deflection, and reflection; and also the physical cause of this property; 

 I hasten to the natural phenomena, the explanation of which depends on the 

 property, whose existence and nature we have been investigating; and for the 

 sake of conciseness and order, we shall rank the phenomena under a division 

 similar to that under which we laid down the principles, beginning with those 

 appearances which are explicable on the principles of flexion. 



J . It is observable, that when a body is exposed in the sun's light, so as to cast 

 a sliadovv, and another body is approached to it, either between the sun and it, 

 or the shadow and it, or on the same line with it, the shadow of the one body 

 comes out a considerable way, and meets tliat of the other. Now it is evident, 

 that when the bodies are held at a sufhcient distance from each other, a penumbra 



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