Opticks 1717 - Up

2.7
great& Refra&ion, mufi be more refrangible
than thofe which go to the lower end T, unlefs
the inequality of RefraCtion be cafual.
This Image or Sp&rum I? T was coloured,
being red at its leait refra&ed end T, and violet
at its mofi refra&ed end P, and yellow
green and blue in the intermediate Spaces..
.Which agrees with the firfi Propofitioy ,,** that
Lights which differ in Colour do allb dukr in
Kefrai$bility, The length of the Image in the
, foregomg Experiments I meafured from, the
faintefi and outmofi red at one end, to the
fainteit and outmoit blue at the other end, excepting
only a little Penumbra, whole breadth
fcarce exceeded a quarter. of an Inch, as was
a:;lid above.
Ex~P~. 4. In the Sun’s beam which was propagated
into the Room through the hole in the
Window-fhut, at the di,rtance of ibme Feet
from the hole, I held the Prifrn in fuch a poiflure
that its Axis might be perpendicular to
that beam. Then I looked through the Prifm
zlpon the hole, and turning the Prifm to and
fro about its Axis to make the Image of the
hole afcend and d&end, when bctwecn its
two contrar Motions it feemed fiationary, I
itopp’d the P %fm that the RefraCtions of both
iides of the refraAing Angle might be equal to
each other ) as in the former Experiment. In
this Situation of the Prifm viewing through it
the faid hole, I obferved the length of its refra8ed
Image to be many times greater than
its breadth, and that the nlofi ref%aAed part
thereof agpeared . violet, the leafi refraAed red,
the