[Illustration: FIG. 21.]

_Illustration._ In the twenty-first Figure, ABC is the first Prism, BC
its Base, B and C its equal Angles at the Base, each of 45 Degrees, A
its rectangular Vertex, FM a beam of the Sun's Light let into a dark
Room through a hole F one third part of an Inch broad, M its Incidence
on the Base of the Prism, MG a less refracted Ray, MH a more refracted