20 PHILOSOPHICAL TRANSACTIONS. [anNO 1672. 



1 . Whether the unequal refractions made without respect to any inequality 

 of incidence, be caused by the different refrangibility of several rays ; or by the 

 splitting, breaking or dissipating the same ray into diverging parts ? 



2. Whether there be more than two sorts of colours ? 



3. Whether whiteness be a mixture of all colours? 



The first of these quaeries you may find already determined by an experiment 

 in my former letter ; the design of which was to show, that the length of the 

 coloured image proceeded not from any unevenness in the glass, or any other 

 contingent irregularity in the refractions. Amongst other irregularities, I 

 know not what is more obvious, to suspect, than a fortuitous dilating and 

 spreading of light, after some such manner as Descartes has described in his 

 sethereal refractions, for explicating the tail of a comet ; or as the animadversor 

 now supposes to be effected by the splitting and rarefying of his aethereal pulses. 

 And to prevent the suspicion of any such irregularities, I told you that I re- 

 fracted the light contrarywise with two prisms successively, to destroy thereby 

 the regular effects of the first prism by the second, and to discover the irregular 

 effects by augmenting them with iterated refractions. Now, amongst other 

 irregularities, if the first prism had spread and dissipated every ray into an in- 

 definite number of diverging parts, the second should in like manner have 

 spread and dissipated every one of those parts into a further indefinite number, 

 whereby the images would have been still more dilated, contrary to the event. 

 And this ought to have happened, because those linear diverging parts depend 

 not on one another for the manner of their refraction, but are every one of them 

 as truly and completely rays as the whole was, before its incidence ; as may ap- 

 pear by intercepting them severally. 



The reasonableness of this proceeding will perhaps better appear by acquaint- 

 ing you with this further circumstance. 1 sometimes placed the second prism 

 in a position transverse to the first, on design to try, if it would make the long 

 image become four-square, by refractions crossing those that had drawn the 

 round image into a long one. For if, amongst other irregularities, the re- 

 fraction of the first prism did, by splitting, dilate a linear ray into a superficial, 

 the cross refractions of that second prism ought, by further splitting, to dilate 

 and draw that superficial ray into a pyramidal solid. But, upon trial, I found it 

 otherwise; the image being as regularly oblong as before, and inclined to both 

 the prisms at an angle of 45 degrees. 



I tried also all other positions of the second prism, by turning the ends about 

 its middle part; and in no case could observe any such irregularity. The image 

 was ever alike inclined to both prisms, its breadth answering to the sun's dia- 



