THE PLANETS, AKE THEY INHABITED 



Fig. 



seen from them severally; since the apparent diameter must 

 decrease in exactly the same proportion as the distance from 

 the sun increases, and vice versa. In this way we find that 

 the sun, as seen from the four planets, has the relative magni- 

 tudes shown in fig. 2, where E being taken to represent the 

 disc of the sun, as seen from the Earth, M will be its disc as 

 seen from Mercury, V, as seen from Venus, and M', as seen 

 from Mars. 



The brightness of the sun's light at Mercury will be greater 

 than at the Earth, in the same proportion, as M is greater than 

 E, and its light at Mars will be less bright 

 than at the Earth, in the same ratio as that 

 in which M' is less than E. It might, there- 

 fore, be concluded that the light at Mars 

 would be too feeble, and the light at Mercury 

 too intense for vision. 



13. A slight consideration of the structure 

 and functions of the eye will, however, 

 demonstrate how easily such difficulties may 

 be removed. The perception of light which 

 any creature possessing that organ acquires, 

 depends (cceteris paribus) upon the mag- 

 nitude of the circular aperture or foramen, 

 in front of the eye, called the pupil, which 

 has, externally, the appearance of a circular 

 black spot ; but which is, in reality, a cir- 

 cular hole through which the light is 

 admitted to the interior of the chamber 

 of vision, there to affect the membranous 

 coating which transmits its influence to the 

 brain and causes the sensation. 



This will be better understood by refer- 

 ence to the annexed figures, 3 and 4, the 

 former representing the external form and appearance of 

 the eye, and the latter a section of the eye-ball, made in a 

 horizontal plane through the dotted line A B. The line P 

 (fig. 3), points to the pupil, I to the iris, a coloured ring 

 surrounding the pupil ; and W to the white of the eye. In 

 fig. 4, P points to the pupil, I to the iris, and N and to a 

 membranous coat full of nerves and blood-vessels which lines the 

 inside of the eye-ball. The light, entering from M G through 

 the pupil, and passing through the internal humours of the eye, 

 which are perfectly transparent, strikes on that membranous 

 coating and acts upon it in such a manner as to produce a 

 perception. The apparent brightness of the light will obviously 

 8 



