PROPOSITIONS.

(one at each) refracting those two beams of Light to the opposite Wall of the Chamber, in such manner that the two colour'd Images PT and MN which they there painted were joined end to end and lay in one straight Line, the red end T of the one touching the blue end M of the other. For if these two refracted Beams were again by a third Prism DH placed cross to the two first, refracted sideways, and the Spectrums thereby translated to some other part of the Wall of the Chamber, suppose the Spectrum PT to pt and the Spectrum MN to mn, these translated Spectrums pt and mn would not lie in one straight Line with their ends contiguous as before, but be broken off from one another and become parallel, the blue end m of the Image mn being by a greater Refraction translated farther from its former place MT, than the red end t of the other Image pt from the same place MT; which puts the Proposition past Dispute. And this happens whether the third Prism DH be placed immediately after the two first, or at a great distance from them, so that the Light refracted in the two first Prisms be either white and circular, or coloured and oblong when it falls on the third.