30 Things not generally Known. 



some experiments by Lord Brougham on the sunbeam, in which, 

 by placing the edge of a sharp knife just within the limit of 

 the light, the ray was inflected from its previous direction, and 

 coloured red ; and when another knife was placed on the op- 

 posite side, it was deflected, and the colour was blue. These 

 experiments (says Mr. Hunt) seem to confirm Sir Isaac New- 

 ton's theory, that light is a fluid emitted from the sun. 



THE UNDULATORY SCALE OF LIGHT. 



The white light of the sun is well known to be composed of 

 several coloured rays ; or rather, according to the theory of 

 undulations, when the rate at which a ray vibrates is altered, 

 a different sensation is produced upon the optic nerve. The 

 analytical examination of this question shows that to produce 

 a red colour the ray of light must give 37,640 undulations in 

 an inch, and 458,000,000,000,000 in a second. Yellow light re- 

 quires 44,000 undulations in an inch, and 535,000,000,000,000 

 in a second ; whilst the effect of blue results from 51,110 un- 

 dulations within an inch, and 622,000,000,000,000 of waves in 

 a second of time. Hunt's Poetry of Science. 



VISIBILITY OF OBJECTS. 



In terrestrial objects, the form, no less than the modes of 

 illumination, determines the magnitude of the smallest angle 

 of vision for the naked eye. Adams very correctly observed 

 that a long and slender staff can be seen at a much greater 

 distance than a square whose sides are equal to the diameter of 

 the staff. A stripe may be distinguished at a greater distance 

 than a spot, even when both are of the same diameter. 



The minimum optical visual angle at which terrestrial ob- 

 jects can be recognised by the naked eye has been gradually 

 estimated lower and lower, from the time when Robert Hooke 

 fixed it exactly at a full minute, and Tobias Meyer required 

 34" to perceive a black speck on white paper, to the period 

 of Leuwenhoeck's experiments with spiders' threads, which are 

 visible to ordinary sight at an angle of 4" 7. In Hueck's most 

 accurate experiments on the problem of the movement of the 

 crystalline lens, white lines on a black ground were seen at an 

 angle of 1"'2 ; a spider's thread at 0"*6 ; and a fine glistening 

 wire at scarcely 0"*2. 



Humboldt, when at Chillo, near Quito, where the crests of the 

 volcano of Pichincha lay at a horizontal distance of 90,000 feet, was 

 much struck by the circumstance that the Indians standing near distin- 

 guished the figure of Bonpland (then on an expedition to the volcano), 

 as a white point moving on the black basaltic sides of the rock, sooner 

 than Humboldt could discover him with a telescope. Bonplaud was 

 enveloped in a white cotton poncho : assuming the breadth across the 

 shoulders to vary from three to five feet, according as the mantle clung 



