[53] My friend, Mr WILLIAM SWAN, carefully examined, by his new and ingenious method, described in the Edinburgh New Philosophical Journal, January 1844, several specimens of the St Gobain glass (which is now used in the manufacture of the lenses), and found its refractive index to be 1·51793, the _difference_ between the greatest and least values being only 0·00109. ~Refraction.~ A ray of light, in passing _obliquely_ from one transparent body into another of different density, experiences at the point of the intersection of the common surface of the two planes, a sudden change of direction, to which the name of _refraction_ has naturally been given, in connection with the most familiar instance of the phenomenon, which is exhibited by a straight ruler with one half plunged into a basin of water while the other remains in the air. The ruler no longer appears straight, but seems to be _bent_ or _broken_ at the point where it enters the water. It may not be out of place to call attention to the laws which regulate the change of direction in the incident light, which are _three_ in number. 1. Incidence and refraction, in uncrystallized media of homogeneous structure such as glass, always occur in a plane perpendicular to that of the refracting surface. 2. In the same substances, the angle formed with the perpendicular by the ray at its entering the surface of the second medium, has to the angle which it makes with the normal after it has entered the surface, such a relation, that their sines have a fixed ratio, which is called the _refractive index_. When a ray falls normally on the surface of any substance, it suffers no refraction. 3. The effect of passing from a rare to a dense medium, as from air into water or glass, is to make the angle of _refraction_ less than the angle of _incidence_; and those angles are measured with reference to a normal to the plane which separates the media at the point of incidence. The converse phenomenon, of course, takes place in the passage from a dense to a rare medium, in which case the angle of _incidence_ is less than the angle of _refraction_. To this rule there are a few exceptions; for there are certain combustible bodies, such as diamond, whose refractive powers are much greater than other substances of equal density. [Illustration: Fig. 47.] The diagram (fig. 47) will serve to render those laws more intelligible. Let a ray of light _a_ O meet a surface of water _n m_ at O, it will be immediately bent into the direction O _a′_; and if, from the centre O, we describe any circle, and draw a line _b_ O _b′_, perpendicular to _nm_; then _ab_ and _a′ b′_, perpendiculars drawn to the normal _bb′_, from the points _a_ and _a′_ where the circle cuts the incident and refracted rays, will be the sines of the angle of incidence _b_ O _a_, and of the angle of refraction _b′_ O _a′_, and the ratio of those sines to each other, or _b a_ ------- _b′ a′_