In this, as we know now, Newton was wrong; the length of the band, the spectrum, is not proportional to the average or mean deviation ; it is possible to have two spectra of the same length in which the deviations are markedly different, thus if two such prisms are mounted with their vertices in opposite directions, the light emerging from the second will be achromatic, largely free from colour, but will be deviated from its original path.
Newton's experiments on colour, made in all probability in order to study a defect of the telescope, led him to the belief that the defect was incurable. By putting a divergence lens behind the converging lens of the object glass the colours could again be combined, but, so he concluded, his rays would all be made parallel to their original direction, and would no longer converge to form a real image, which could be magnified by the eyepiece ; the tele scope could not be made to give a colourless (achromatic) image. This belief he retained throughout his life, but it was wrong. Dispersion and the separation of the colours, are not proportional to deviation.
Shortly after Newton's death Chester Moor Hall invented an achromatic telescope and by 1733 had made several, and in 1758 Dollond the optician took up the matter and constructed satis factory achromatic lenses.
But his mistake had important consequences; a real image, one, that is, through which the light actually passes, of a distant object can be formed by reflexion at a concave mirror, and since the laws of reflexion, unlike those of refraction through a transparent medium, are independent of colour, the reflected image is the same colour as the object, the rainbow band is there no longer; chromatic aberration, as it is called, is no more a disturbing factor. This then led Newton to make his reflecting telescope in which a concave mirror takes the place of the object glass ; the first telescope was made in 1668; the second was sent by him to the Royal Society in December 1671 shortly before his election as a Fellow.
Discussions as to the Optics paper lasted until 1675. "I was so persecuted" he writes in December of that year "with discussions arising out of my theory of light that I blamed my own impru dence for parting with so substantial a blessing as my quiet to run after a shadow." But they had their advantage. They led him to investigate other effects of colour, to enquire how light was produced and to develop the emission or corpuscular theory of light, according to which light is due to the emission by a lumi nous body of a host of tiny particles travelling in empty space with a speed of 190,000 miles per second; the laws of reflexion and refraction were developed on mechanical principles, aided only by a supplementary hypothesis as to why, when falling on a transparent surface, some of the particles are reflected—bent back into the medium whence they have come—others are refracted, along a new path inclined to the old, into the medium towards which they are travelling. It is a consequence of this theory that
light travels more quickly in a dense medium such as glass than in air. The theory was also applied to explain the colours seen when light is reflected from a thin film, a soap film, or the thin layer of air between a convex lens of large radius and a flat reflecting surface on which it rests; in this case when viewed in reflected light of a definite colour a series of dark and light rings circling round a central black spot is seen. Newton determined the law connecting the radius of a bright ring and the colour of the light and since the radius depends on the colour, the bright rings for the various colours, when white light is used, will be different and the observer will see a series of coloured rings sur rounding the black central spot.
Hooke was again a critic ; in his Micrographia (1664) he had adopted the wave theory of light, due originally to Huyghens, according to which light is energy transmitted by wave motion through a medium pervading space, the universal ether, and had endeavoured to explain, but without success, rectilinear propaga tion, reflexion and refraction as well as dispersion and the colours of thin plates. Hooke's arguments were vague and carried no con viction to a mind like Newton's; the latter sought a mechanical explanation for all he observed. Newton in his explanation of the reflection and refraction of the corpuscles of a light centre made use of the idea of a wave in an ethereal medium; he rejected Huyghens' theory and thought little of Hooke's attempts at ex planation, and so for a hundred years or more Newton's theory held the field. In 1804 Thomas Young, Professor at the Royal Institution, London, established the principle of interference by which he showed that under certain conditions two parcels of light from identical sources falling on a screen could produce a series of bright and dark bands. Along certain lines on the screen there is a maximum of brightness; along others intermediate be tween these there is darkness. From this and the brilliant work of the French genius Augustin Fresnel, a few years later, came the explanation on the wave theory of all the phenomena of light as then observed.