No greater influence on the art attended the next attempt at a reflector, by Isaac Newton (1643-1727). This was an early outcome of his notable discovery of the dispersion of light by prisms, which led him to despair of improving refracting telescopes and turned his mind to reflectors.
Unhappily in an experiment to determine whether refraction and dispersion were proportional he committed the singular blunder of raising the refractive index of a water-filled prism to equality with glass by dissolving sugar of lead in it. Without realizing the impropriety of thus varying two quite unknown quantities at once in his crucial experiment, he promptly jumped to the conclusion that refraction and dispersion varied in exact proportion in all substances, so that if two prisms or lenses dispersed light to the same extent they must also equally refract it. It would be interesting to know just how the fact of his bungling was passed along to posterity. As a naïve apologist once remarked, it was not to be found in his “Optics.” But Sir David Brewster and Sir John Herschel, both staunch admirers of the great philosopher, state the fact very positively. If one may hazard a guess it crept out at Cambridge and was passed along, perhaps to Sir William Herschel, via the unpublished history of research that is rich in picturesque details of the mare’s nests of science. At all events a mistake with a great name behind it carries far, and the result was to delay the production of the achromatic telescope by some three quarters of a century.
Turning from refractors he presented to the Royal Society just after his election as Fellow in 1672, the little six-inch model of his device which was received with acclamation and then lay on the shelf without making the slightest impression on the art, for full half a century.
Newton, by dropping the notion of direct view through the tube, hit upon by far the simplest way of getting the image outside it, by a plane mirror a little inside focus and inclined at 45°, but injudiciously abandoned the parabolic mirror of his original paper on dispersion. His invention therefore as actually made public was of the combination with a spherical concave mirror of a plane mirror of elliptical form at 45°, a construction which in later papers he defended as fully adequate.[5]
Fig. 13.—Newton’s Model of his Reflector.
His error in judgment doubtless came from lack of practical astronomical experience, for he assumed that the whole real trouble with existing telescopes was chromatic aberration, which in fact worried the observer little more than the faults due to other causes, since the very low luminosity toward the ends of the spectrum enormously lessens the indistinctness due to dispersion.
As a matter of fact the long focus objective of small aperture did very creditable work, and its errors would not compare unfavorably with those of a spherical concave mirror of the wide aperture planned by Newton. Had he actually made one of his telescopes of fair dimensions and power the definition would infallibly have been wrecked by the aberrations due to spherical figure.[6]
