40 
Dr. Young's Lecture on 
u reflection, there is propagated a kind of fainter ray—," and, 
M by reason of the time spent in passing and repassing,— this 
ff£ fainter pulse comes behind the" former reflected ££ pulse ; so 
ic that hereby, (the surfaces being so near together that the eye 
“ cannot discriminate them from one,) this confused or duplicated 
“ pulse, whose strongest part precedes, and whose weakest fol- 
££ lows, does produce on the retina, the sensation of a yellow. 
“ If these surfaces are further removed asunder, the weaker 
“ pulse may become coincident with the" reflection of the 
S£ second," or next following pulse, from the first surface, “ and 
“ lagg behind that also, and be coincident with the third, 
a fourth, fifth, sixth, seventh, or eighth — ; so that, if there be 
<£ a thin transparent body, that from the greatest thinness requi- 
u site to produce colours, does by degrees grow to the greatest 
£C thickness,— the colours shall be so often repeated, as the 
ee weaker pulse does lose paces with its primary or first pulse, 
“ and is coincident with a" subsequent “ pulse. And this, as 
“ it is coincident, or follows from the first hypothesis I took of 
“ colours, so upon experiment have I found it in multitudes of 
“instances that seem to prove it." (P. 65 — 67.) This was 
printed about seven years before any of Newton's experiments 
were made. We are informed by Newton, that Hooke was 
afterwards disposed to adopt his ££ suggestion" of the nature of 
colours ; and yet it does not appear that Hooke ever applied that 
improvement to his explanation of these phenomena, or inquired 
into the necessary consequence of a change of obliquity, upon 
his original supposition, otherwise he could not but have dis- 
covered a striking coincidence with the measures laid down by 
Newton from experiment. All former attempts to explain the 
colours of thin plates, have either proceeded on suppositions 
