OPTICS. 



65 



6. The animal kingdom also contri- 

 butes its aid in support of the same 

 theory. The choroid coat of the dog 

 and other animals, which produces the 

 blue, green, and red reflexions from the 

 eye of the living animal, retains the 

 same faculty after death. When the 

 choroid coat dries, it becomes black, and 

 the colours disappear. We have found, 

 however, that after remaining dry for 

 nearly ten years, their colours could still 

 be developed by moisture. The black 

 passed instantly into a brilliant blue, the 

 blue into green, and the green into 

 greenish yellow. 



5. Colours of the atmosphere. As 

 the earth is surrounded with an atmo- 

 sphere varying in density from the sur- 

 face of the globe, where it is a maxi- 

 mum, to the height of about 45 miles, 

 where it is extremely rare, and just able 

 to reflect the rays of the setting sun, the 

 rays of the sun, moon, and stars are re- 

 fracted into curve lines, unless when 

 they are incident upon it perpendicularly. 

 Hence the apparent altitude of the celes- 

 tial bodies is always greater than their 

 real altitude, and they appear above the 

 horizon when they are actually below it. 



But while the solar rays traverse the 

 earth's atmosphere, they suffer another 

 change from the resisting medium which 

 they encounter. When the sun, or any 

 of fhe heavenly bodies, are considerably 

 elevated above the horizon, their light 

 is transmitted to the earth without any 

 perceptible change ; but when these 

 bodies are near the horizon, their light 

 must pass through a long tract of air, 

 and is considerably modified before it 

 reaches the eye of the observer. The 

 momentum of the red, or greatest re- 

 frangible rays, being greater than the 

 momentum "of the violet, or least re- 

 frangible rays, the former will force their 

 way through the resisting medium, 

 while the latter will be either reflected 

 or absorbed. A white beam of light, 

 therefore, will be deprived of a portion 

 of its blue rays by its horizontal passage 

 through the atmosphere, and the re- 

 sulting colour will be either orange or 

 red, according to the quantity of the 

 least refrangible rays that have been 

 stopt in their course. Hence the rich 

 and brilliant hue with which nature is 

 gilded by the setting sun ; hence the 

 plowing red which tinges the morning 

 and evening clouds; and hence the 

 sober purple of twilight which they as- 

 sume when their ruddy glare is tempered 

 by the reflected azure of the sky. 



We have already seen that the red 

 rays penetrate through the atmosphere, 

 while the blue rays, less able to sur- 

 mount the resistance which they meet, 

 are reflected or absorbed in their pas- 

 sage. It is to this cause that we must 

 ascribe the blue colour of the sky, 

 and the bright azure which tinges the 

 mountains of the distant landscape. 



As we ascend in the atmosphere, the 

 deepness of the blue tinge gradually 

 dies away ; and to the aeronaut who has 

 soared above the denser strata, or to 

 the traveller who has ascended the Alps 

 or the Andes, the sky appears of a deep 

 black, while the blue rays find a ready 

 passage through the attenuated strata 

 of the atmosphere. It is owing to the 

 same cause, that the diver at the bottom 

 of the sea is surrounded with the red 

 li^ht which has pierced through the 

 superincumbent fluid, and that the blue 

 rays are reflected from the surface of 

 the ocean. Were it not for the reflect- 

 ing power of the air, and of the clouds 

 which float in the lower regions of the 

 atmosphere, we should be involved in 

 total darkness by the setting of the sun, 

 and all the objects around us would suffer 

 a total eclipse by every cloud that passed 

 over his disk. It is to the multiplied 

 reflections which the light of the sun 

 suffers in the atmosphere, that we are 

 indebted for the light of day, when the 

 earth is enveloped with impenetrable 

 clouds. 



From the same cause arises the sober 

 hue of the morning and evening twilight, 

 which increases as we recede from the 

 equator, till it blesses with perpetual 

 day the inhabitants of the polar regions. 



The cause which we have assigned 

 for the blue light of the sky, and which 

 was, we believe, first given by Bou- 

 guer, though a very probable one, still 

 required the evidence of demonstration. 

 In examining this light, Dr. Brewster 

 found that a great portion of it was 

 polarized ; and hence it follows, that it 

 has suffered reflexion. M. Saussure 

 found that the intensity of the blue 

 colour increased with the height of the 

 observer above the sea ; and it has 

 been observed by others, that the inten- 

 sity diminishes as the quantity of aqueous 

 vapour is increased. In order to mea- 

 sure this intensity, M. Saussure contrived 

 an instrument called a Cyanometer.* A 

 circular band of thick paper or paste- 

 board is divided into 5 i parts, each of 



* From two Greek words signifying a measure and 

 llucness. 



