740 PHILOSOPHICAL TRANSACTIONS. [aNNO 1672, 



Mr. Nejfton^s Answer to the foregoing Letter. N" 85, p. 5014. 



Translated from the Latin. 



In the observations of the Rev. F. Pardies, one can hardly determine whe- 

 ther there is more of humanity and candour, in allowing my arguments their 

 due weight, or of penetration and genius in starting objections. And doubtless 

 these are very proper qualifications in researches after truth. But to proceed, 

 F. Pardies says, that the length of the coloured image can be explained, with- 

 out having recourse to the divers refrangibility of the rays of light ; as suppose 

 by the hypothesis of F. Grimaldi, viz. by a diffusion of light, which is supposed 

 to be a certain substance put into very rapid motion ; or by Mr. Hook's hypo- 

 thesis, by a diffusion and expansion of undulations ; which, being formed in the 

 aether by lucid bodies, is propagated every way. To which may be added the 

 hypothesis of Descartes, in which a similar diffusion of conatus, or pression of the 

 globules, may be conceived, like as is supposed in accounting for the tails of 

 comets. And the same diffusion or expansion may be devised according to any 

 other hypothesis, in which light is supposed to be a power, action, quality, or 

 certain substance emitted every way from luminous bodies. 



In answer to this, it is to be observed that the doctrine which I explained 

 concerning refraction and colours, consists only in certain properties of light, 

 without regarding any hypothesis, by which those properties might be explain- 

 ed. For the best and safest method of philosophizing seems to be, first to in- 

 quire diligently into the properties of things, and establishing those properties 

 by experiments ; and then to proceed more slowly to hypotheses for the expla- 

 nation of them. For hypotheses should be subservient only in explaining the 

 properties of things, but not assumed in determining them ; unless so far as 

 they may furnish experiments. For if the possibility of hypotheses is to be the 

 test of the truth and reality of things, I see not how certainty can be obtained 

 in any science; since numerous hypotheses may be devised, which shall seem to 

 overcome new difBculties. Hence it has been here thought necessary to lay 

 aside all hypotheses, as foreign to the purpose, that the force of the objection 

 should be abstractedly considered, and receive a more full and general answer. 



By light therefore I understand, any being or power of a being, (whether a 

 substance, or any power, action, or quality of it,) which, proceeding directly 

 from a lucid body, is apt to excite vision. And by the rays of light I understand 

 its least or indefinitely small parts, which are independent of each other j such 

 as are all those rays which lucid bodies emit in right lines, either successively or 

 all together. For the collateral as well as the successive parts of light are mde- 



