Striking the glass at point, the intercepted ray reflects to L; diminished by the reflected light, it enters the glass and is refracted toward B; from it is again refracted toward, and diminished by the direct light, it reflects to M; and diminished by the light reflected toward by the second meeting with the air, it finally is refracted toward, and the line which according to experience and the laws of optics is parallel to is that of the ray which makes us see the less illuminated of the small circles, and the ray derives its light only from the light reflected from the ray while passing from the glass to the air below.