OPTICS. 



CHAPTER II. 



THE WATER TELESCOPE.— IRIS OF THE EYE.— MAGIC LANTERN. 

 —THE SPECTROSCOPE.- THE THAUMATROPE. 



Limits to Sight in the Water. — Effect of a Ripple. — The Eyes under Water.— The 

 Water Telescope, its Structure and Mode of Use. — Gyrinus, or Whirlwig- 

 beetle, and its Double Set of Eyes. — The Iris of the Eye, aud its Double Set of 

 Contractile Fibres. — Cotterill's Lock and its Structure. — The Magic Lantern 

 and its Principle. — Chinese Shadows. — Spectre of the Brocken. — An Adven- 

 ture in Wiltshire. — Effect of the Halo. — The Spectroscope. — Its Structure 

 explained. — A Star on fire. — Motes in the Sunbeams. — Bessemer Steel made 

 by aid of the Spectroscope. — Absorption Bands. — Detection of Blood. — A 

 Man's Life saved by the Spectroscope. — The Pocket Spectroscope. — The 

 Rainbow, Dewdrop, Soap-bubble, Opal, and Pearl. — The Thaumatrope. — 

 Structure of the Retina. — Complementary Colours. — The Zoetrope and 

 Chromatrope. — Wheel Animalcules and their Structure. — An Optical Delu- 

 sion. 



The Water Telescope. 



EVERY one who has watched the movements of the various 

 creatures which live below the surface of the water is 

 aware how entirely dependent he is on the unruffled character 

 of that surface. No matter how clear the water may be, the 

 least ruffling of the surface will effectually shut out all sight : — 



"But if a stone the gentle sea divide, 

 Swift rippling circles rush on every side, 

 And glimmering fragments of a broken sun, 

 Banks, trees, and skies in thick disorder run." 



And there is an end of the observations. If, however, the eyes 

 can penetrate below the surface, the ruffling is of little con- 

 sequence, so long as the water is clear. Consequently, when- 

 ever the top of the bank is sufficiently near the water, it is 

 possible to continue the observations by lying down, and 

 immersing the head above the eyes. This plan, however, is 



u 2 



