In order that the ethereal medium may produce the fits of easy reflection and transmission, he conceives that its vibrations must be swifter than light. He computes its elasticity to be 490,000,000,000 times greater than that of air, in proportion to its density, and about 600,000,000 times more rare than water, from which he infers that the resistance which it would oppose to the motions of the planets would not be sensible in 10,000 years. He considers that the functions of vision and hearing may be performed chiefly by the vibrations of this medium, executed in the bottom of the eye, or in the auditory nerve by the rays of light, and propagated through the solid, pellucid, and uniform capillamenta of the optic or auditory nerves into the place of sensation; and he is of opinion that animal motion may be performed by the vibrations of the same medium, excited in the brain by the power of the will, and propagated from thence by the solid, pellucid, and uniform capillamenta of the nerves into the muscles for contracting and dilating them. In the registers of the Royal Society there exist several letters[117] on the excitation of electricity in glass, which were occasioned by an experiment of this kind having been mentioned in Sir Isaac’s hypothesis of light. The society had ordered the experiment to be tried at their meeting of the 16th December, 1675; but, in order to secure its success, Mr. Oldenburg wrote to Sir Isaac for a more particular account of it. Sir Isaac being thus “put upon recollecting himself a little farther about it,” remembers that he made the experiment with a glass fixed at the distance of the 1/3d of an inch from one end of a brass hoop, and only the 1/8th of an inch from the other. Small pieces of thin paper were then laid upon the table; when the glass was laid above them and rubbed, the pieces of paper leaped from the one part of the glass to the other, and twirled about in the air. Notwithstanding this explicit account of the experiment, it entirely failed at the Royal Society, and the secretary was desired to request the loan of Sir Isaac’s apparatus, and to inquire whether or not he had secured the papers from being moved by the air, which might have somewhere stole in. In a letter, dated 21st December, Sir Isaac recommended to the society to rub the glass “with stuff whose threads may rake its surface, and, if that will not do, to rub it with the fingers’ ends to and fro, and knock them as often upon the glass.” These directions enabled the society to succeed with the experiment on the 13th January, 1676, when they used a scrubbing brush of short hog’s bristles, and the heft of a knife made with whalebone.