Drop centering device

1. A variable-focus lens comprising:

a wall made of an insulating material;

a drop of an insulating liquid arranged on an area of a first surface of the wall;

a conductive liquid covering the first surface and the drop, the insulating and

conductive liquids being non-miscible, having different optical indexes and substantially the same density; and

an electrode arranged on a second surface of said wall,

wherein the drop is centered in a truncated cone centered on an axis perpendicular to the first surface, the bottom of which is pierced with a centered hole, and the insulating material wall is transparent and covers said hole.

2. The variable-focus lens of claim 1 , whereby the truncated cone causes a flattening of a lens surface formed at an interface of the insulating and conductive liquids.

3. The variable-focus lens of claim 1 , whereby a curvature radius of a boundary between the insulating and conductive liquids increases with the slope of the truncated cone.

4. A variable-focus lens comprising:

a wall made of an insulating material and defining a cavity having at least one sidewall;

a drop of an insulating liquid arranged on a first surface of the wall;

a conductive liquid covering the drop, the insulating and conductive liquids being non-miscible, having different optical indexes and substantially the same density; and

an electrode arranged on a second surface of the wall,

wherein the wall has a shape for centering the drop,

wherein the cavity comprises a truncated cone, centered on an axis perpendicular to the first surface, and wherein the drop is centered in the truncated cone.

5. The variable-focus lens of claim 4 , whereby the truncated cone causes a flattening of a lens surface formed at an interface of the insulating and conductive liquids.

6. The variable-focus lens of claim 4 , whereby a curvature radius of a boundary between the insulating and conductive liquids increases with the slope of the truncated cone.

7. A variable-focus lens comprising:

a wall made of an insulating material and defining a cavity having at least one sidewall;

a drop of an insulating liquid arranged on a first surface of the wall;

a conductive liquid covering the drop, wherein the insulating and conductive liquids are non-miscible, having different optical indexes; and

an electrode arranged on a second surface of the wall,

wherein the wall has a shape for centering the drop,

wherein the cavity comprises a truncated cone, centered on an axis perpendicular to the first surface, and wherein the drop is centered in the truncated cone.

8. The variable-focus lens of claim 7 , whereby the truncated cone causes a flattening of a lens surface formed at an interface of the insulating and conductive liquids.

9. The variable-focus lens of claim 7 , whereby a curvature radius of a boundary between the insulating and conductive liquids increases with the slope of the truncated cone.

10. A variable-focus lens comprising:

a cavity having a side surface flared relative to an optical axis;

a conductive liquid covering a drop of an insulating liquid disposed in the cavity, wherein the conductive and insulating liquids are non-miscible, having different optical indexes and substantially the same density, and the conductive and insulating liquids making a lens surface at an interface of the conductive and insulating liquids; and

an electrode provided on a second surface of the cavity and having a hole encompassing the optical axis, the electrode being shaped to encompass the insulating liquid,

wherein the flared side surface of the cavity comprises a truncated cone.

11. The variable-focus lens of claim 10 , whereby the truncated cone causes a flattening of the lens surface formed at the interface of the insulating and conductive liquids.

12. The variable-focus lens of claim 10 , whereby a curvature radius of a boundary between the insulating and conductive liquids increases with the slope of the truncated cone.

13. An optical system in which light is converged by a lens element, said optical system comprising:

an optical element constituting a portion of said optical system, said optical element including a container having a side surface inclined at a predetermined angle from an optical axis;

first and second liquids contained in said container, wherein said first and second liquids differ from each other in refractive index, said first and second liquids exist without mixing with each other, and said first and second liquids make a boundary between said first and second liquids having a rounded shape; and

an electrode formed in such a place as to avoid interference with passage of a bundle of rays incident upon said optical element, said electrode being ring-shaped to surround at least one of said first and second liquids,

wherein the side surface of the container is inclined.

14. The optical system of claim 13 , wherein the side surface of the container is inclined so that a curvature radius of the boundary between the first and second liquids increases.

15. The optical system of claim 13 , wherein the first and second liquids have substantially the same density.