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the action upon it of proper muscles provided for that purpose. The quantity of light admitted through the pupil to the retina is increased or diminished in the proportion of the area of the pupil, which increases and diminishes in proportion to the square of its diameter; a very small variation of which, therefore, produces a very considerable proportionate variation of the quantity of light admitted.
9. If a person, after remaining for some time in a room dimly lighted, pass suddenly into one which is strongly illuminated, he will become instantly sensible of pain in the retina, and will involuntarily close his eyes. After a short time, however, he will be enabled to open them and look around with impunity.
10. The cause of this is easily explained. In the dimlylighted room the pupil was widely expanded, to collect the largest quantity possible of the faint light, so that a sufficient quantity might be received by the retina to produce a sensible perception of the surrounding objects. On passing into the strongly-illuminated room the expanded pupil admits so much of the intense light as to act painfully on the retina before there is time for the iris to adjust itself so as to contract the aperture of the pupil. After a short interval, however, this adjustment is made, and the area of the pupil being diminished in the same proportion as the intensity of the light to which it is exposed has been augmented in passing from one room to the other, the action upon the retina is proportionally mitigated, so that the can regard without pain the surrounding objects. 11. The reverse of all this takes place when the eye
suddenly passes from strong to feeble illumination. The pupil contracted when exposed to the strong light is not sufficiently open to admit the rays of feeble light necessary to produce visual perception, and for some time the surrounding objects are invisible. When, however, the proper muscular apparatus has had time to act upon the eye so as to enlarge the pupil, the rays are admitted in greater quantity, and the surrounding objects begin to be perceived.
12. These phenomena are beautifully expressed by the lines of Moore:
“Thus when the lamp that lighted
The traveler, at first goes out,
And lingers on in fear and doubt.
“But soon the prospect clearing,
In cloudless starlight on he treads,
As that light which heaven sheds.”
13. Nevertheless, there is a point in this which demands some explanation. It is implied in these lines that the source of nocturnal illumination is chiefly, if not exclusively, starlight. This has been in a great measure disproved by Arago, who shows that there must be some other source of nocturnal illumination than that of the stars. On nights, for example, which are thickly clouded there is sometimes a stronger light than on those in which the firmament is clear and serene. From this and other circumstances Arago argues that there must be some power of illumination in the clouds or in the atmosphere independently of the light which proceeds from the stars. This is a point, however, the full development of which would require more space and time than we can spare for it on the present occasion.
14. In another of Moore's poems we find the following beautiful lines:
Thus, Mary, be but thou my own;
While brighter eyes unheeded play,
That bless my home and guide my way.”
15. This is not only beautiful poetry, but sound astronomy. The distances of the stars are many hundreds of millions of times greater than that of the moon, but their actual splendor is in many cases greater than that of the
Thus it has been shown by calculations made upon observations which appear to admit of no doubt, that the star Sirius, commonly called the Dog Star, is a sun one hundred and forty-six and a half times more splendid than that which illuminates our system. Its distance, however, is so enormous that, the actual light which it sheds upon our firmament is less than the five-thousand-millionth part of the sun's light.
16. Another star, which is the principal one in the constellation of the Centaur, has been ascertained to be a sun whose splendor is two and one-third times greater than that of ours, but, owing to its enormous distance, the light which it sheds in our firmament is twenty-two thousand million times less than that of the sun. Sir John Herschel found by exact photometric measurement that the light shed upon us by this star was twenty-seven thousand four hundred and eight times less than the light of the full
17. Shakspeare imputes to the cricket the sense of hearing:
“I will tell it softly; young crickets shall not hear me.” This was long considered as a scientific blunder on the part of the poet, the most eminent naturalists having maintained that insects in general have no sense of hearing. Brunelli, an Italian naturalist, however, has demonstrated that the cricket at least has that sense. Several of these insects, which he shut up in a chamber, continued their usual crinking or chirping the whole day, except at moments when he alarmed them by suddenly knocking at the door. The noise always produced a temporary silence on their part. He contrived to imitate their sounds so well that the whole party responded in a chorus, but were instantly silenced on his knocking at the door.
18. The female glow-worm, which emits the phosphorescent light familiar to all who have dwelt in warm climates, remains comparatively stationary to await the approach of her mate, attracted to her by the light which she holds out to him-a circumstance of which Moore has availed himself with his usual felicity :
“ Beautiful as is the light
Her mate to her green bower at night.”
“True! Therefore doth Heaven divide
N the sea and at the Hogue, sixteen hundred ninety-two,
And, the thirty-first of May, helter-skelter through the blue, Like a crowd of frightened porpoises a shoal of sharks pursue, Came crowding ship on ship to St. Malo on the Rance,
With the English fleet in view.
'Twas the squadron that escaped, with the victor in full chase, First :::d foremost of the drove, in his great ship, Damfreville;
Close on him fled, great and small,
Twenty-two good ships in all;
Help the winners of a race!
Here's the English can and will!”
Then the pilots of the place put out brisk and leaped on board; “Why, what hope or chance have ships like these to pass ?”
laughed they; “Rocks to starboard, rocks to port, all the passage scarred and
scored, Shall the 'Formidable’ here, with her twelve and eighty guns,
Think to make the river-mouth by the single narrow way, Trust to enter where 't is ticklish for a craft of twenty tons,
And with flow at fall beside ?
Now 't is slackest ebb of tide.
Not a ship will leave thé bay!”
Then was called a council straight;
Brief and bitter the debate; “Here's the English at our heels; would you have them tako
in tow All that's left us of the fleet, linked together stern and bow,