A Problem Is Addressed
In 1672, when Isaac Newton published his celebrated paper on light and colors, he showed that white light is a mixture of colored light and every color had its own degree of refraction.
The result was that any curved lens would decompose white light into the colors of the spectrum, each of which comes to a focus at a different point on the optical axis. This effect, which became known as chromatic aberration, resulted in a central image being surrounded by circles of different colors.
He therefore decided to try a mirror. He cast a two-inch mirror blank of speculum metal (basically copper with some tin) and ground it into spherical curvature. He placed it in the bottom of a tube and caught the reflected rays on a 45° secondary mirror which reflected the image into a convex ocular lens outside the tube .