touching his Theory of Light and Colours. 189 



vision, and the difference of colours, as also their harmony 

 and discord, I shall leave to their consideration who may 

 think it worth their endeavour to apply this hypothesis to 

 the solution of phenomena*." Were I to assume an hypo- 

 thesis, it should be this, if propounded more generally so as 

 not to determine what light is, further than that it is some- 

 thing or other capable of exciting vibrations in the aether ; for 

 thus it will become so general and comprehensive of other 

 hypotheses as to leave little room for new ones to be invented ; 

 and therefore because I have observed the heads of some 

 great virtuosos to run much upon hypotheses, as if my dis- 

 courses wanted an hypothesis to explain them by, and found 

 that some, when I could not make them take my meaning 

 when I spake of the nature of light and colours abstractedly, 

 have readily apprehended it when I illustrated my discourse 

 by an hypothesis ; for this reason I have here thought fit to 

 send you a description of the circumstances of this hypothesis, 

 as much tending to the illustration of the papers I herewith 

 send you ; and though I shall not assume either this or any 

 other hypothesis, not thinking it necessary to concern myself 

 whether the properties of light discovered by me be explained 

 by this, or by Mr. Hook's, or any other hypothesis capable of 

 explaining them, yet while I am describing this, I shall, some- 

 times to avoid circumlocution and to represent it more conve- 

 niently, speak of it as if I assumed it and propounded it to be 

 believed. This I thought fit to express, that no man may 

 confound this with my other discourses, or measure the cer- 

 tainty of the one by the other, or think me obliged to answer 

 objections against this script; for I desire to decline being 

 involved in such troublesome, insignificant disputes. 



But to proceed to the hypothesis. — 1. It is to be supposed 

 therein, that there is an aetherial medium, much of the same 

 constitution with air, but far rarer, subtiler, and more strongly 

 elastic. Of the existence of this medium, the motion of a 

 pendulum in a glass exhausted of air almost as quickly as in the 

 open air is no inconsiderable arguments But it is not to be 



* Letter to O. Camb. July 11, 1672. Phil. Trans. No. 88. p. 5087-8. 



t Either we must presume that a word is here omitted in the manu- 

 script, and that the sentence should stand thus — " the motion of a pen- 

 dulum 'ceasing' in a glass exhausted of air almost as quickly as in open 

 air," and suppose this statement made on the authority of Boyle's experi- 

 ments, in which the difference of the times of vibration in the two cases was 

 "scarce sensible" (New Exp. Phys.-Mech., Exp. 26), — or we must conclude 

 that Newton had already, iu 1675, arrived, by experiments of his own, at 

 the same conclusion to which Derham and Hawksbee came in 1704, that 

 in consequence of the extension of the arcs of vibration, the vibrations, 

 though quicker in their rates, are in their times " slower in the exhausted 



