'The object-metal was two inches broad, and about one-third part of an inch thick, to keep it from bending. Had it magnified but 30 or 25 times, it would have made the Object appear more brisk and pleasant'. Yet Objects appeared much darker in it than in the Glass, and that partly because more Light was lost by Reflexion in the Metal, than by Refraction in the Glass, and partly because my Instrument was overcharged. By comparing it with a pretty good Perspective of four Feet in length, made with a concave Eye-glass, I could read at a greater distance with my own Instrument than with the Glass. For this Circle being placed here, stopp'd much of the erroneous Light, which other wise would have disturbed the Vision. The concave Metal bore an Aperture of an Inch and a third part, but the Aperture was limited not by an Opake Circle, covering the limb of the Metal round about, but be an opake Circle, placed between the Eyeglass and the Eye, and perforated in the middle with a little round hole for the Rays to pass through to the Eye. By another way of measuring I found it magnified 35 times. The Eye-glass was Plano-convex, and the diameter of the Sphere to which the convex side was ground was about 1/5 of an Inch, or a little less, and by consequence it magnified between 30 and 40 times. 'The diameter of the sphere to which the Metal was ground concave was about 25 English Inches, and by consequence the length of the Instrument about six Inches and a quarter. He also made all the tube, mount, and fittings.

This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He added to his reflector what is the hallmark of the design of a 'Newtonian telescope', a secondary 'diagonal' mirror near the primary mirror's focus to reflect the image at 90° angle to an eyepiece mounted on the side of the telescope. He chose a spherical shape for his mirror instead of a parabola to simplify construction: he had satisfied himself that the chromatic, and not the spherical aberration, formed the chief faults of refracting telescopes. He devised means for shaping and grinding the mirror and may have been the first to use a pitch lap to polish the optical surface. To create the primary mirror Newton used a custom composition of metal consisting of six parts copper to two parts tin, an early composition of speculum metal. The telescope he constructed used mirrors as the objective which bypass that problem. He had concluded that the lens of any refracting telescope would suffer from the dispersion of light into colours ( chromatic aberration). Isaac Newton built his reflecting telescope as a proof for his theory that white light is composed of a spectrum of colours. 19th century drawing of Newton's reflector

Opticks by Isaac Newton