[ 736 ] 
or increafed tlie angle between the glafs plates, till I 
found the two contrary refradtions to be equal j 
which I difcovered by viewing an objedt thro this 
double prifm ; which, when it appeared neither raifed 
nor depreffed, I was fatisfied, that the refradtions 
were equal, and that the emergent rays were parallel 
to the incident. 
Now, according to the prevailing opinion, the ob- 
jedt fhould have appeared thro’ this double prifm 
quite of its natural colour ; for if the difference of 
refrangibility had been equal in the two equal re- 
fradtions, they would have redtified each other : but 
the experiment fully proved the fallacy of this re- 
ceived opinion, by (hewing the divergency of the 
light by the prifm to be almoft double of that by 
the water j for the objedt, tho’ not at all refradted, 
was yet as much infedted with prifmatic colours, as 
if it had been feen thro’ a glafs wedge only, whofe 
refradting angle was near 30 degrees. 
N. B. This experiment will be readily perceived 
to be the fame as that which Sir lfaac Newton 
mentions * ; but how it comes to differ fo very 
remarkably in the refult, I fhall not take upon 
me to account for ; but will only add, that I 
ufed all poffible precaution and care in the pro- 
cefs, and that 1 keep the apparatus by me to 
evince the truth of what I write, whenever I 
may be properly required fo to do. 
I plainly faw then, that if the refradting angle 
of the water-veffel could have admitted of a fufficient 
* Book. I. Part ii. Prop. 3. Experiment 8. of his Optics. 
increafe 
