Page:Optics.djvu/25



1. the nature of light, very little is known with any certainty; fortunately it is not at all necessary in mathematical enquiries about it, to establish any thing about its constitution. The science of Optics reposes on three Laws, as they are technically termed, which depend for their proof upon Observation and Induction.

2.In the first place, as it is observed that an object cannot be discerned if it be placed directly behind another not transparent, we conclude that the action of light takes place in straight lines. These straight lines are called rays, and are the sole object of discussion in the following Treatise.

3.When a small beam of light admitted through a hole in the shutter into a dark room falls upon a plane polished surface, such as that of a common mirror, it is observed to be suddenly bent back, or reflected, according to the technical phrase, and as it has otherwise the same appearance as before, we conclude that each ray of light is bent at the point where it meets the surface, or that more properly for each ray that existed in the beam, we have now two, an incident and a reflected ray meeting in the surface.

Observation leads us to conclude that these rays are invariably in the same plane, and that they as invariably make equal angles with the reflecting surface, or with a line perpendicular to it at the point of reflection: the angles which the incident and reflected rays respectively make with this perpendicular are called the angles of incidence and reflection.

4.If again we present to the beam of light above-mentioned a very thick plate of glass or a vessel of water, or any other transparent substance, we shall find that part of the light is reflected on