A cone may be regarded as a succession of cylinders of different diameters graduating into one another by exceedingly small steps, so that if a short enough portion be considered, its curvature at any point may be regarded as cylindrical.  A lens with one side plane and the other ground on a conical tool is therefore a concave cylindrical lens varying in concavity at different parts according to the diameter of the cone at the corresponding part.  Two such lenses mounted with axes parallel and with curvatures varying in opposite directions produce a compound cylindrical lens, whose refraction in the direction of the axes is zero, and whose refraction in the meridian at right angles to this is at any point the sum of the refractions of the two lenses.  This sum is nearly constant for a considerable distance along the axis so long as the same position of the lenses is maintained.  If the lenses be slid one over the other in the direction of their axes, this sum changes, and we have a varying cylindrical lens.  The lens is graduated by marking on the frame the relative position of the lenses when cylindrical lenses of known power are neutralized.

Lenses were exhibited to the Royal Society, London, varying from to -6 DCy, and from to +6 DCy.

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THE LAWS OF THE ABSORPTION OF LIGHT IN CRYSTALS.

By H. BECQUEREL.

1.  The absorption spectrum observed through a crystal varies with the direction of the rectilinear luminous vibration which propagates itself in this crystal. 2.  The bands or rays observed through the same crystal have, in the spectrum, fixed positions, their intensity alone varying. 3.  For a given band or ray there exist in the crystal three rectangular directions of symmetry, according to one of which the band generally disappears, so that for a suitable direction of the luminous vibrations the crystal no longer absorbs the radiations corresponding to the region of the spectrum where the band question appeared.  These three directions may be called the principal directions of absorption, relative to this band. 4.  In the orthorhombic crystals, by a necessary consequence of crystalline symmetry, the principal directions of absorption of all the bands coincide with the three axes of symmetry.  We may thus observe three principal absorption spectra.  In uniaxial crystals the number of absorption spectra is reduced to two. 5.  In clinorhombic crystals one of the principal directions of absorption of each crystal coincides with the only axis of symmetry; the two other principal rectangular directions of each band may be found variously disposed in the plane normal to this axis.  Most commonly these principal directions are very near to the principal corresponding directions of optical elasticity. 6.  In various crystals the characters of the absorption phenomena differ strikingly from those which we might expect to find after an examination of the optical