38 Things not generally Known. 



each eye separately, are similar ; and the appearance to the 

 two eyes is precisely the same as when the object is seen by 

 one eye only. There is, in such case, no difference between 

 the visual appearance of an object in relief and its perspective 

 projection ou a plane surface ; hence pictorial representations 

 of distant objects, when those circumstances which would pre- 

 vent or disturb the illusion are carefully excluded, may be 

 rendered such perfect resemblances of the objects they are in- 

 tended to represent as to be mistaken for them. The Diorama 

 is an instance of this. Professor W/ieatstone; Philosophical 

 Transactions, 1838. 



CURIOUS OPTICAL EFFECTS AT THE CAPE. 



Sir John Herschel, in his observatory at Feldhausen, at the 

 base of the Table Mountain, witnessed several curious optical 

 effects, arising from peculiar conditions of the atmosphere in- 

 cident to the climate of the Cape. In the hot season "the 

 nights are for the most part superb ;" but occasionally, during 

 the excessive heat and dryness of the sandy plains, " the op- 

 tical tranquillity of the air" is greatly disturbed. In some 

 cases, the images of the stars are violently dilated into nebular 

 balls or puffs of 15' in diameter; on other occasions they form 

 " soft, round, quiet pellets of 3' or 4' diameter," resembling 

 planetary nebulas. In the cooler months the tranquillity of 

 the image and the sharpness of vision are such, that hardly any 

 limit is set to magnifying power but that which arises from 

 the aberration of the specula. On occasions like these, optical 

 phenomena of extraordinary splendour are produced by viewing 

 a bright star through a diaphragm of cardboard or zinc pierced 

 in regular patterns of circular holes by machinery : these phe- 

 nomena surprise and delight every person that sees them. 

 When close double stars are viewed with the telescope, with a 

 diaphragm in the form of an equilateral triangle, the discs of 

 the two stars, which are exact circles, have a clearness and 

 perfection almost incredible. 



THE TELESCOPE AND THE MICROSCOPE. 



So singular is the position of the Telescope and the Micro- 

 scope among the great inventions of the age, that no other 

 process but that which they embody could make the slightest 

 approximation to the secrets which they disclose. The steam- 

 engine might have been imperfectly replaced by an air or an 

 ether-engine ; and a highly elastic fluid might have been, and 

 may yet be, found, which shall impel the "rapid car," or drag 

 the merchant- ship over the globe. The electric telegraph, 

 now so perfect and unerring, might have spoken to us in the 



