102 TRANSACTIONS OF THE [FEB. 21, 



tlie geometrical principle that two surfaces cannot fit each other 

 in all positions unless both are perfectly spherical. The tool of 

 the optician is a very simple affair, being nothing more than a 

 plate of iron somewhat larger, perhaps a fourth, than the lens to 

 be ground to the corresponding curvature. In order to insure 

 its changing to fit the glass, it is covered on the interior with a 

 coating of pitch from an eighth to a quarter of an inch thick. 

 This material is admirably adapted to the purpose because it gives 

 way certainly, though very slowly, to the pressure of the glass. 

 In order that it may have room to change its form, grooves are 

 cut through it in both directions, so as to leave it in the form of 

 squares, like those on a chess-board. 



'* It is then sprinkled over with rouge, moistened with water, 

 and gently warmed. The roughly ground lens is then placed 

 upon it, and moved from side to side. The direction of the 

 motion is slightly changed with every stroke, so that after a 

 dozen or so of strokes the lines of motion will lie in every direc- 

 tion on the tool. This change of direction is most readily and 

 easily effected by the operator slowly walking around as he 

 polishes, at the same time the lens is to be slowly turned around 

 either in the opposite direction or more rapidly yet in the same 

 direction, so that the strokes of the polisher shall cross the lens 

 in all directions. This double motion insures every part of the 

 lens coming into contact with every part of the polisher, and 

 moving over it in every direction. Then whatever parts either 

 of the lens or of the polisher maybe too high to form a spherical 

 surface will be gradually worn down, thus securing the jierfect 

 sphericity of both. 



''When the polishing is done by machinery, which is the cus- 

 tom in Europe, with large lenses, the polisher is slid back and 

 forth over the lens by means of a crank attached to a revolving 

 wheel. The polisher is at the same time slowly revolved around 

 a pivot at its centre, which pivot the crank works into, and the 

 glass below it is slowly turned in the opposite direction. Thus 

 the same effect is produced as in the other system. Those who 

 practise this method claim that by thus using machinery the 

 conditions of a uniform polish for every part of the surface can 

 be more perfectly fulfilled than by a hand motion. The results, 



