( 70O ) 
the Angles of 
degr. mn. 
the fecond JPrifm. 
S54 o 
<62 12 
C63 48 
the Lengths cf 
the Image. 
71 
lol 
You may perceive, that the length of the Images inrefpcft 
of the angles that made them , arefomething greater in the fe- 
cond prifm than in the firft ; but chat was becaufe theglafs^ of 
which the fecond Prifm was made, had the greater refradlive 
power. 
The days in which I made thefe Trials were pretty clear, 
but not fo clear as I defired, and therefore afterwards meeting 
with a day as clear as I defired , I repeated the Experiment 
with the. fecond Prifm, and found the lengths of the Image 
made by its feveral angles to be about ^ of an inch greater than 
before, the meafures being thofe fct down in this Table. 
the Angles 0/ 
degr. min, 
-4he fecond Prifmi 62 12 i ©f 
C63 48 I 1 1 
the Lengths of 
the image. 
7 
Thereafonof this difference I apprehend was, that in the 
clearefl: days the light of the white skies, which dilutes and 
renders invifible the feinteft Colours at the ends of the Image, is 
alittlediminilhed inaclear day, and fo gives leave to the Co- 
lours to -appear to a greater length ; the Suns light at the fame 
time becoming brisker , ,and fo ftrengthning the Colours and 
making the faint ones at the two ends more confplcuous. For 
I have obfcrved, that in days fomething cloudy , whilft the 
Prifm has ftood unmoved at the window, the Image would grow 
a little longer or a little (hotter, accordingly as the Sun was 
more or lefs obfcured by thin Clouds which paflTed over it ; 
the Image being ffiorteft when the Cloud was brighreftand the 
Suns light faioteft. Whence it is cafie to apprehend, that, if 
the light ofUfee Clouds could be quite taken away, fo that the 
Sun 
