742 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672. 



in the present figure. And besides, if the diameter of the hole y be equal to 

 the breadth of the colours, the coloured light will not be diffused lengthwise ; 

 but the image, which is formed by any colour at G or H, will be manifestly 

 circular ; supposing the holes to be circular, and the refraction of the latter 

 prism not to be greater than that of the former, and the rays to be nearly per- 

 pendicular to the obstacle. This shows that the diffusion, above-mentioned, 

 does not arise from the influence or continuity of the undulating matter, or 

 matter put into a rapid motion, or any such like causes, but from a certain law 

 of refractions for every species of rays. But why the image is in one case circu- 

 lar, and in others a little oblong, and how the diffusion of light lengthwise may 

 in any case be diminished at pleasure, I leave to be determined by geometri- 

 cians, and compared with experiments. 



After the properties of light shall, by these and such like experiments, have 

 been sufficiently explored, by considering its rays either as collateral or succes- 

 sive parts of it, of which we have found by their independence that they are 

 distinct from one another; hypotheses are thence to be judged of, and those 

 to be rejected which cannot be reconciled with the phasnomena. But it is an 

 easy matter to accommodate hypotheses to this doctrine. For if any one wish 

 to defend the Cartesian hypothesis, he need only say that the globules are un- 

 equal, or that the pressures of some of the globules are stronger than others, 

 and that hence they become differently refrangible, and proper to excite the 

 sensation of different colours. And thus also according to Hook's hypothesis, 

 it may be said, that some undulations of the aether are larger or denser than 

 others. And so of the rest. For this seems to be the most necessary law and 

 condition of hypotheses, in which natural bodies are supposed to consist of a 

 multitude of corpuscles cohering together, and that from the different particles 

 of lucid bodies, or from the different parts of the same corpuscle, (as they may 

 happen to differ in motion, figure, bulk, or other qualities) unequal pressions, 

 motions, or moved corpuscles, may be propagated every way through the sether, 

 of the confused mixture of which light may be supposed to be constituted. 

 And there can be nothing more difficult in these hypotheses than the contrary 

 supposition. 



As to that aperture or dilatation of the light in the posterior face of the prism, 

 which the Rev. Father supposes to resemble a hole, it is sufficient that no sensi- 

 ble error can arise from it, if any at all. For if a calculation be made precisely 

 according to the observations, the error will be found nothing. For by sub- 

 tracting the diameter of the hole from the length of the image, there will remain 

 that length which the image would have, if the hole before the prism were an 

 indivisible point, and that notwithstanding the aforesaid dilatation of the light 



