Page:Reflections upon ancient and modern learning (IA b3032449x).pdf/240

 ther, but still preserve the same Order till they come to the Brain. The crystalline Humour had already been discovered to be of a Double-Convex Figure, made of Two unequal Segments of Spheres, and not perfectly spherical as the Ancients thought. So that this further Discovery made by Dr. Briggs, shews evidently why all the Parts of the Image are so distinctly carried to the Brain, since every Ray strikes upon a several Filament of the Optick Nerve, and all those strings so struck are moved equably at the same Time. For want of knowing the Nature and Laws of Refraction, which have been exactly stated by Modern Mathematicians, the Ancients discoursed very lamely of Vision. This made Galen think that the crystalline Humour (f) was the Seat of Vision, whose only Use is to refract the Rays, as the known Experiment of a dark Room, with one only Hole to let in Light, through which a most exact Land-skip of every thing without, will be represented in its proper Colours, Heights and Distances, upon a Paper placed in the Focus of the Convex Glass in the Hole, which Experiment is to be found in almost every Book of Opticks, does plainly prove. Since the same thing will appear, if the cry-