[ 735 ] 
by the different refrangibilty of light j or, in other 
words, that however a ray of light might be re- 
fracted backwards and forwards by different me- 
diums, as water, glafs, &c. provided it was fo done, 
that the emergent ray fhould be parallel to the in- 
cident one, it would ever after be white ; and, con- 
verfely, if it ihould come out inclined to the inci- 
dent, it would diverge, and ever after be coloured. 
From which it was natural to infer, that all fpherical 
objeCt-glaffes of telefcopes muff be equally affeCted 
by the different refrangibility of light, in proportion 
to their apertures, whatever material they may be 
formed of. 
But it feems worthy of confideration, that not- 
withftanding this notion has been generally adopted 
as an inconteftable truth, yet it does not leem to 
have been hitherto fo confirmed by evident experi- 
ment, as the nature of fo important a matter juffly 
demands ; and this it was that determined me to 
attempt putting the thing to iffue by the following 
experiment. 
I cemented together two plates of parallel glafs at 
their edges, fo as to form a prifmatic or wedge-like 
veffel, when flopped at the ends or bafes ; and its 
edge being turned downwards, I placed therein a 
glafs prifm with one of its edges upwards, and 
filled up the vacancy with clear water : thus the 
refradion of the prifm was contrived to be contrary 
to that of the water, fo that a ray of light trans- 
mitted thro’ both thefe refracting mediums would 
be refraCted by the difference only between the two 
refractions. Wherefore, as I found the water to re- 
fraCt more or lefs than the glafs prifm, I diminiflied 
or 
2 
