Учебно-методическое пособие по чтению специальной литературы для студентов 1 курса физического факультета. Дроздова И.В - 14 стр.

                                          14

2.   It is a fact however – as Newton and others before him found out – that the
     only way light rays from an object point can come to an exact focus at another
     point is by reflection at a plane, or flat surface. A plane mirror, for example,
     will reflect an image of an object that is perfect and sharp to the last detail. But
     when light rays are bent or refracted at a flat, spherical, or elliptical surface
     between one medium, such as air, and another such as glass, they give images
     that are indistinct.

3.   This optical nuisance is called spherical aberration. Because most telescopes
     before Newton’s generation had spherical lenses, they all suffered from it.
     Even Galileo’s famous telescope was quite a faulty instrument and the strain of
     spherical aberration probably helped to make Galileo blind toward the end of
     his life. A thorough search was being made by the scientific minds of the day
     to try to do away with spherical aberration.

4.   Earlier, the great French scientist, Rene Descartes, thought he had solved the
     difficulty when he suggested that not spherical lenses were needed but
     elliptically shaped ones. Yet the grinding and polishing of such surfaces was a
     difficult, if not possible, task in Newton’s time. Even if the workmen of the
     day had been able to produce the lenses suggested by Descartes, there would
     have been little improvement on the problem. However, it was not aberration
     alone that lay at the heart of the difficulty. Actually the fault lay elsewhere, and
     not only Galileo and Descartes, but Kepler too, were unaware of this.

5.   At Woolsthorpe, when he was only twenty-three, Newton had begun
     experimenting with the prism – and had reached the startling conclusion that
     there was no way to improve the telescope of Kepler and Galileo. Instruments
     would have to be build according to an entirely new idea if men were to view
     the heavens with anything like true exactness. Newton had begun to realize
     that these early telescopes were inaccurate not only because of spherical
     aberration, but because of colour.

6.   Think back to Newton’s experiment with the prism at Woolsthorpe. He had
     passed white light through his prism and it had broken down, by refraction,
     into the colours of the spectrum. Naturally the lenses of these early telescopes
     refracted light just as a prism did. This resulted in the blurred colour fringes
     that annoyed Galileo and others when, for example, they focused their
     instruments on a distant star. Their lenses - whatever their shape – simply
     could not produce a sharp, clear image of the star because colours had been
     introduced by refraction.

7.   Newton, however knew that reflection at any surface would not produce colour
     blurring, and he decided to give up the idea of refracting telescopes altogether.
     He would build one on the reflection principle. Newton was not the first to
     think of the idea, however. A few years before, the distinguished Scottish