The manufacture of spherical surfaces for spectacle lenses is very similar in principle to the methods of the instrument maker, but in addition to these there are toric lenses and fused and solid bifocals which require quite different manufacturing techniques. The glass used for single vision lenses is a standard white crown with a refractive index of 1·523 and a v–value 61. A range of tinted glasses, such as Crookes' A1, A2, B and B2, and several other types are used for special prescriptions. The glass-cutting room draws suitable glass from stock to prepare and examine it ready for moulding. The furnace operator stacks the pieces of rough glass at the side of his furnace, and all subsequent handling is done with a pair of foils. Fused bifocals consist of a main lens of crown glass with a small area of flint glass fused to it, thus producing two different powers.