C 174 ] 
cd with violet and red : for a violet ray from the up- 
permost point ot the Sun’s difk will be more refract- 
ed than the other can be ; and a red ray from the 
lowed; part of his difk, will be lefs refraCted than any 
other. 
; If BC, the diftance of the refra&ing furfaces, 
is increaled or diminished ; R V, the diftance of the 
extreme rays, will be increafed or diminished in the 
fame proportion: and if RV approaches very near 
to the aperture O, the colours will become imper- 
ceptible. 
4. To reunite thefe rays, we may place another 
medium of the fame refractive power, and of the 
fame thickneSs {be — BC) as in the figure ; fo as the 
rays V v, Rr, &c. may enter its Surface c d at the 
fame angle as they emerged at from CD, or as SO 
entered A B ; and after refraCtion at the point 0 ot 
the furface ab, to which they converge, they will be 
reunited into os the continuation of SO, in a pencil 
every way like the incident pencil SO, excepting that 
the lieht will have been fomewhat weakened in its 
paftage through the media. 
5. Other things remaining, let the thicknefs ot 
the fecond medium be cp> lets than cb or CB, the 
furface parallel to cd being pe-, and the emergent rays 
utr will be indeed parallel to the incident as formerly, 
but the fpedtrum will fall below the place of the 
lcrcen where SO or os would fall. It will like wife 
be coloured, as the rays were not yet united at the 
point 0. If the thicknefs be greater than cb^ the 
ipeCtrum will fall above the line SO os , and the violet 
and red, after their interfeClion in 0 , will have chang- 
ed Sides. 
6 Other 
