Huygens was fortunately placed to make a fundamental conjectural leap. He invented the first cascade theory for the propagation of light. This cascade or chain reaction theory was based on intimate knowledge and association with the behaviour of light as it passed through lenses at various stages of production, from a lens blnk to the finished spherical, concave or convex lens.
At each stage certain colourful light patterns would appear and these were called chromatic aberrations, but they were exactly the same as Moire patterns. Such patterns could be seen when the concentric rings of a lense in process of grinding were seen at odd angles. Huygens was the first to design so called complex lens systems of more than one lense. These overlapping concentric rings of grinding produced complex Moite patterns. The patterns seemed to persist no matter how snootly the lenses were polished.
According to Newton these were caused by dispersion and refraction of the dispersed ray should cure the issue, but it did not. In place of the coloured lines dark rings appeared called Newtons rings. Newtons assertion that dispersion was the same as diffraction was not correct. Grimsldis diffraction still happened to the Newtonian refracted ray.
Huygens had to start thinking in terms of pinhole cameras , but how could a pin hole exist in a clear , translucent and transparent lens?
How could Moire patterns form in clear transparent and translucent glass, where there was nothing to block the lights path of propagation?
The first thought was that dispersion into coloured rays somehow affected the refraction angles for the lense surface. In practical terms radii of curvature were used not refraction angles when grinding a lens. So was the compromise creating refraction angles that were not prismatic for every colour?
Certsinly yes, and every attempt was made to remove these refraction anomalies. It was found that polishing with an oil seemed to correct for these problems. But how?
In the process the light seemed to be subtly changed. And the dark rings were still evident in the refracted light.
Huygens, like Newton, used a spray of rays to represent the dispersed chromatic rays, and he recognised that the refracting boundary had to be a reversed prism of the dispersing surface to recombine the dispersed rays. But not all the dispersed rays could be being refracted. Then he thought in terms of diffraction.
Maybe diffraction was not identical to dispersion! Grimaldi showed fainter bands of light around a pinhole camera image. This soon lead to additional rays being added to the dispersed rays. The pont of incidence became a point of production of secondary rays, these secondary rays fanned out in a wider arc than the dispersed rays. It was these extra rays that were the complement to the dispersed rays, and these were not refracted by the lens design.
Could they be the missing light in the dark rings, diffracted elsewhere out of or in the lens?
Huygens may not have precisely followed this train of thought, but he did realise thst dispersion suddenly made the point of incidence into a kind of pin hole camera, and he did realise some of the light was missing..
The Moire patterns certainly gave him an intuitive notion of what we call interference patterns caused by diffraction or splitting of light. His experience was that light split by dispersion, but he struggled to connect the diffracted chromatic light with the Moire patterns becausebhevstill thought in terms of refractive focussing. A pinhole camera works by diffractive focussing. Diffractive focussing would have to wait for Fresnel to discover.
However, the wave theory of sound now came into focus, and this would further elaborate his ideas on light propagation. So far we got to a point in the lense surface where the ray is incident being what is now called optically active, and dispersing the light ray. And another set of poinst in the opposite surface refracting each chromatic light ray to a combining focus point. The lens design was to get this to happen within the lens so that all the light going in comes out in its original structure..
How did he go from these optically active points to every point on a wave front of a spherical nature being optically active?
I think the influence is Descartes andvNewtons wave theory of sound, and a person named Pardies
When reading what others write about Huygens it is weii to remember that they probably have not read Huygens Theoria.
Most recount what others thought he might have said!
Transparent Calcite was to play a crucial role in both the propagation theory and the polarisation of light theory.
The etymology of refraction
Descartes pressure disturbance travelling in the plenum
I have to make some corrections in the light of new information. But it is clear that Descartes rational or inspired thinking was the basis of much of modern physics. Newton now has a place as an advocate of empiricism. Newtons effect was to force redefinitions of several terms used descriptively and especially the term refraction which appears to have had no bearing on the nature of light but was a geometrical term to describe what happened to a presumed straight line ray. Such a line was fractioned and refractiomed. Thus it was observed that a line or ray of light was fractured 2 times from its course, but not separated from the ray. Thus it was a continuous line that was made up of straight line segments not curved.
In projective geometry, therefore a straight edge could be used to draw the path of the ray , and if the angles of fraction and refraction were known then a straight line could be drawn to indicate the path of the ray inside a material.
Newton therefore used dispersion for the first fractioning of the ray and refraction for the second fractioning of the dispersed rays. . By using a reversing prism he was able to recombine the dispersed rays again by a fraction refraction process..
Grimaldis diffraction therefore meant that a piece of light was completely broken off! In terms of rays this was inconceivable . You could always progress a ray between any 2 points. Grimaldi was a corpuscularism and therefore explained it in terms of corpuscles that were fragmented. These corpuscles then travelled in ray like paths ballistically.
It would seem that Grimaldis theory influenced Newton away from the pressure theory of Descartes to a corpuscular theory.. That is diffraction was understood by Newton as the light being split into corpuscles.
In that light his discovery of chromatic rays confirmed him in his view thatP light could be split into corpuscles and these corpuscles could be spread out.or dispersed, showing they too travelled ballistically in rays.
Newton realised that his spectrum appeared continuous o he divided it. This served to enhance the notion of separate corpuscles of colour, but there was no empirical evidence for this. However Grimaldies diffraction lines appeared in other experiments within he colour bnd and so he was convinced that hs partition was justified.
Diffraction was therefore used by newtonto define splitting light into corpuscles and into individual rays. Dispersion was also included in the notion of diffraction.
Thus diffraction created the 2 competing theories of propagation in light. It is poetic that Newyons work in wave theory of sound should inspire the competing theory to sharpen its description and demonstrate its empirical basis.