the Theory of Light and Colours. 27 
termination, instead of the right line BG, will assume the form 
CH; since the loss of force must be more considerable near to C 
than at greater distances. This line corresponds with the boun- 
dary of the shadow in Newton's first observation, Fig. 1 ; and 
it is much more probable that such a dissipation of light was 
the cause of the increase of the shadow in that observation, 
than that it was owing to the action of the inflecting atmo- 
sphere, which must have extended a thirtieth of an inch each 
way in order to produce it ; especially when it is considered 
that the shadow was not diminished by surrounding the hair 
with a denser medium than air, which must in all probability 
have weakened and contracted its inflecting atmosphere. In 
other circumstances, the lateral divergence might appear to in- 
crease, instead of diminishing, the breadth of the beam. 
As the subject of this proposition has always been esteemed 
the most difficult part of the undulatory system, it will be 
proper to examine here the objections which Newton has 
grounded upon it. 
“ To me, the fundamental supposition itself seems impossible ; 
“ namely, that the waves or vibrations of any fluid can, like the 
“ rays of light, be propagated in straight lines, without a con- 
“ tinual and very extravagant spreading and bending every 
“ way into the quiescent medium, where they are terminated 
“ by it. I mistake, if there be not both experiment and demon- 
“ stration to the contrary." (Phil. Trans. VII. 5089, Abr. I. 
146. Nov. 1672.) 
“ Motus omnis per fluidum propagatus divergit a recto tra- 
u mite in spatia immota." 
“ Quoniam medium ibi," in the middle of an undulation 
E 2 
