  Newton developed theories of optics and chemistry that were atomistic in the weak sense that they sought to explain optical and chemical properties by invoking interacting particles lying beyond the range of observation. However, the particles were not ultimate.  Newton's position on the least parts of chemical substances was similar to that of Boyle and other mechanical philosophers. They were regarded as made up of a hierarchy of yet smaller particles. So long as the smallest particles were held together by forces, the problem of the ontological status of the forces remained. The least parts of chemicals in Newton's theory were akin to natural minima with the added detail that their action was due to attractive and repulsive forces. As far as the particles of light in Newton's optics are concerned, whether they were ultimate or not, they too acted by way of forces and also suffered fits of easy reflection and easy refraction, the latter being used to explain interference phenomena such as Newton’s rings and why a ray incident on a boundary between two refracting media can be partially reflected and partially transmitted.