516 HISTORY OF SCIENCE. 



veniences in its management. But Foucault, taking advantage of the 

 simple chemical process by which Liebig deposited upon glass sur- 

 faces an adherent film of brilliantly polished silver, proposed to con- 

 struct the mirrors of glass, ground and polished, of the required form, 

 and silvered in front. The film of silver retains its brilliancy longer 

 than speculum metal, and ha* this advantage, that whew required it 

 can be renewed in all its original brilliancy. A splendid reflector, 

 having a silvered glass speculum 4 feet in diameter, was a few years 

 ago erected at the Observatory of Paris on this plan. The grinding of 



FIG. 235. THE KALEIDOSCOPE. 



the glass mirrors to a true form is a work of much delicacy, and re- 

 quires special skill. In this country many tele 1 ^ opes with glass mirrrors 

 ground by Mr. With, of Hereford, have been mounted by Mr. Brown- 

 ing, the London optician, who has also fitted up a popular model of 

 this kind of astronomical telescope. 



As a pendent to the foregoing notice of the improvements which 

 have been effected in the two noblest instruments of optics, we may 

 glance briefly at two optical contrivances which have had an astonishing 

 popularity. They are perhaps too well known to need description. 

 One is the kaleidoscope, which was at one time the most popular of 

 scientific toys. Hundreds of thousands were sold in one month in 

 London and Paris when the kaleidoscope was first made in a cheap 

 form. The instrument, though accounted but as a child's toy, is really 

 a very elegant illustration of the principle of symmetry by reflection. 



