Newton explained that the differing refractions of colors in
sunlight were responsible for the colors of primary and secondary
rainbows, thus completing the inquiry begun by Alhazen in 1025.
Isaac Newton’s first reflecting telescope, finished in
late 1668, had an object mirror with a diameter of 2 inches, a
length of 6 and a quarter inches, and a small diagonal mirror to
reflect the image to an eyepiece at the side of the telescope. He
observed that it magnified 35 times.
William Herschel and William Parson followed Newton’s
design and built reflecting telescopes with speculum mirrors, but
with larger and larger diameters to gather more and more light.
See also in The book of science:
Readings on wikipedia: