[ *73 ] 
XXXI. Rules and Examples for limiting the 
Cafes in which the Rays of refraSted Eight 
may be reunited into a colourlefs Pencil : In 
a Eetter from P. Murdoch, M. A. and 
F.R.S. to Robert Symmer, Efq\ ERA. 
Jan. 3, 1763. 
Read June 2, j i. T ET SO, a fmall pencil of the 
I763 ‘ J j folar light, pafs through the re- 
fracting medium ABCD [Tab. XIV. Fig. J.] whofe 
oppolite furfaces, reprefented by AB, CD, are paral- 
lel planes : then the violet rays OV, will, in the 
fecond refraCtion into the air, emerge parallel to the 
red ; for both will be parallel to the incident ray S O, 
and confequently to each other: that is, V n u will be 
parallel to R r, as is plain from the common prin- 
ciples of Optics. 
2. If the light after its emergence is received on a 
fcreen placed any where beyond RV, it will be tinged 
with violet on the fide V*u, and with red towards Rr: 
and if the incident pencil SO is exceeding fmall, all 
the intermediate colours will be feen in the fame or- 
der as when light is refradted by a prifm. 
But if the incident pencil is not very fmall j or if 
the luminous body from which the rays are tranf- 
mitted through a fmall opening at O, has a confide- 
rable breadth, like that of the Sun’s difk; then lo 
many rays of every kind will mix towards the mid- 
dle of the fpedrum as to produce a pure white ; but 
at the extremities V v and R r, it will ftill be ting 
