^ ( 3^77 ) 
I then proceeded to examin more critically, what might be ef- 
fefted by the difference of the incidence of Rays coming from di' 
vers parts of the Sun ; and to that end, meafured the feverai iioes 
and angles, belonging to the Image. Its diftance from the hole 
or Prilrae was 2 2 foot 5 its utmoft length i^j^ inches 5 its breadth 
2-1' 5 the diameter of the hole ^ of an inch 5 the angle, with the 
Rays, tending towards the middle of the image^ made with thofe 
lines, in which they would have proceeded without refraftion^was 
44deg*j6'. And the vertical Angle of the Prifme 5 63 deg. 12\ 
Alfo the Refra(3:ions 00 both fides the Prifme, that is^ of the Iri-. 
cident, and Emergent Rays, were as near, as I could make them^ 
equa!,and confequently about 54 deg. 4'. Aod the Rays fell per- 
pendicularly upoa the walK Now fubdiicSing the diameter of the 
hole from the lengl|i and brcadch of the Image, there remains 13 
Inches the length, ^d 2|- the breadth, comprehended by thofe 
Rays, which pafFed through the center of the faid hole, and con^ 
fequendy the angle of the hole, which that breadth fubtended, 
was about 31', anfwerabl^ to the Suns Diameter; but the aogle^ 
which its length fubtended 5 was more then five fuch diameters^ 
namely 2 deg. 49'. 
Having made thefe obfervations, I firft computed from them 
the refra5:ive power of that glafs, and found ic meafured by the 
ratio the fines, 20 to 51. And then, h^xhztratio^ I computed 
the Refractions of two Rays flowing from oppofice parts pf the 
Sun's difcusj fo as to differ 51' in their obliquity of Incidence, and 
foundj that the emergent Rays fliould have compreheoaed an 
angle of about 3 1', as they did, before they were incidents 
Butbetaufe this computation was founded on the Hypothefis 
of the proportionality of the /7;^/ of Incidence, and Refradion, 
which though by my own Experience I could not imagine to be 
fo erroneous , as to make that Angle but 3 1 which in reality was 
2 deg. 49' 5 yetmycuriofitycaufed me again to take my Prifme. 
And having placed it at ray window, as before^ I obferved,that by 
turning it a little about its axis to and £ o, fo as to vary its obli- 
quity to the light, more then an angle of 4 or 5 degrees5the Co- 
lours were not thereby fenfibly tranflated from their place on the 
wall, and confequently by that variation of Incidence, the quan- 
tity of Refraftion was not fenfibly varied. By this Experiment 
therefore, as well as by the former computation , it was evident, 
that the difference of the Incidence of Rays, flowing fromdivers 
Gggg2 parts 
