To the first of these objections it is answered--that so vast is the tenuity of light, that it utterly exceeds the power of conception: the most delicate instrument having never been certainly put in motion by the impulse of the accumulated sun-beams. It has been calculated that in the space of 385,130,000 Egyptian years, (of 360 days,) the sun would lose only the 1/1,217,420th of his bulk from the continual efflux of his light. And, therefore, if in 385 _millions_ of years the sun’s diminution would be so extremely small, it would be altogether insensible during the comparatively short period of five or six thousand years. To the second objection it is replied--that the particles of light are so extremely rare that their distance from one another is incomparably greater than their diameters--that all objections of this kind vanish when we attend to the continuation of the impression upon the retina, and to the small number of luminous particles which are on that account necessary for producing constant vision. For it appears, from the accurate experiments of M. D’Arcy, that the impression of light upon the retina continues _eight thirds_, and as a particle of light would move through 26,000 miles in that time, constant vision would be maintained by a succession of luminous particles twenty-six thousand miles distant from each other. Without attempting to decide on the merits of these two hypotheses, I shall leave the reader to adopt that opinion which he may judge to be attended with the fewest difficulties, and proceed to illustrate some of the _properties of light_:--and in the discussion of this subject, I shall generally adhere to the terms employed by those who have adopted the hypothesis of the _emanation_ of light.