There is yet another Circumstance or two of this Experiment by which it becomes still more plain and convincing. Let the second Prism DH [in Fig. 16.] be placed not immediately after the first, but at some distance from it; suppose in the mid-way between it and the Wall on which the oblong Spectrum PT is cast, so that the Light from the first Prism may fall upon it in the form of an oblong Spectrum πτ parallel to this second Prism, and be refracted sideways to form the oblong Spectrum pt upon the Wall. And you will find as before, that this Spectrum pt is inclined to that Spectrum PT, which the first Prism forms alone without the second; the blue ends P and p being farther distant from one another than the red ones T and t, and by consequence that the Rays which go to the blue end π of the Image πτ, and which therefore suffer the greatest Refraction in the first Prism, are again in the second Prism more refracted than the rest.
The same thing I try'd also by letting the Sun's Light into a dark Room through two little round holes F and φ [in Fig. 17.] made in the Window, and with two parallel Prisms ABC and αβγ placed at those holes