VOL. LXXXII.] PHILOSOPHICAL TRANSACTIONS. 233 



copes, and which, in several of the phases, resembles the interior uneven border 

 of the moon, as it appears to the naked eye, or to a power magnifying from 1 to 

 4 times. 



But in a clear and calm atmosphere, and with a high magnifying power, it is 

 truly pleasing to see, after the eye is accustomed to it, how the whole of the 

 terminating border, even to the farther extremities of the cusps, vanishes gra- 

 dually, and becomes at last so faint, that in the day time, and where there are 

 any inequalities, it insensibly loses itself in the colour of the sky. Such striking 

 diminutions of light have I seen repeatedly with my 4-feet reflector, with a power 

 magnifying 280 times, and my 7-feet reflector, with a 370 magnifying power; 

 and particularly on the 20th of November, 1791, when with a power of l6l, I 

 saw the light of the terminator dwindle away, and appearing, for a breadth of 

 about 1 or li seconds, almost as grey as the ash-coloured spots on the moon. 



Those who are at all acquainted with the theory of light, need hardly be re- 

 minded, that on an illuminated spherical surface of a planet, the light will ever 

 appear fainter towards its border, in proportion as the angle between the incident 

 ray from the sun and the said surface becomes smaller. But what here claims our 

 particular notice, is the singular circumstance that, except in the planet Mercury, 

 a similar diminution of light is not observed in so sensible a degree on any of the 

 other planets of our solar system, our earth only excepted. 



Mars, Jupiter, and Saturn, cannot, indeed, on account of their great dis- 

 tance, exhibit on our globe the variable phases of a half, or smaller portion, of 

 an illuminated hemisphere, whence no fair arguments can be derived from those 

 instances: but the comparative appearances of the moon, in this respect, will be 

 thought the more singular if carefully attended to, the marginal diminution of 

 light on this satellite, which however, like Venus, is a sphere illuminated by the 

 sun, not being nearly so perceptible and evident, as that above described. Of 

 this we may fully persuade ourselves, by comparing the appearances of the ter- 

 minating borders of the moon in its falcated phases or quadratures, with the 

 same borders on Venus at the same periodical aspects. Should this striking dif- 

 ference not be reconcileable on our established optical principles, nothing will re- 

 main but the analogy, that, as the density of our atmosphere checks the sun 

 beams the more, the longer they proceed therein in a direction which, after the 

 rise or before the setting of the sun, carries them over a certain tract of land ; 

 and as such a tract, on which the sun at its rising and setting shines from the 

 horizon, is but feebly illuminated; so also is Venus circumstanced with regard to 

 the light it receives from the sun. To compare with some accuracy the intensity 

 of light at the terminating borders on our earth, relatively to its full perpendi- 

 cular illumination, with the same phenomenon on Venus, may, for want of op- 

 portunities to observe both planets at once from a proper distance, as may be done 



VOL. XVU. H H 



