Chemistry and Physics. 57 
remain vertical. By this means the two rods always represent 
respectively the incident and refracted rays, and the index of 
matic and sph aberrations are corrected by a central lens of 
dense glass s lens is nearly three times as thick as the crown- 
glass lenses. The or curves a emispheres, ‘The 
by reflection from the surface of the deep curves e platyscopic 
lens is made of three powers magnifying respectively 15, 20 an 
30 diameters. It possesses the convenien oddington 
lie . P. 
15. New Hlectro-Magnet.—M. V. Sexrrin calls attention to the 
difficult experienced with powerful electric lights from the heat- 
ing of the magnets in their regulators. The effect of this is to burn 
the insulating material and destroy the effect of the magnet by 
— vireuiting it. This difficulty is avoided by a metallic helix 
easily taken to pieces. 
tibly to the touch with even a powerful 
. ’ may be heated to redness without undergo- 
mg sensible change or losing its efficiency. Some further im- 
provements have been introduced into the lator so that 
the luminous point may be moved without extinguishing it, a mat- ~ 
