Microscope objective

1. A microscope dry objective comprising, in order from an object side:

a first lens unit with positive refracting power, having two meniscus lenses, each with a concave surface facing the object side, and at least one positive lens;

a second lens unit with positive refracting power; and

a third lens unit having concave surfaces adjacent and opposite to each other configured as air contact surfaces,

wherein the following conditions are satisfied:

7≦ f

0.5<NA

where f is a focal length, in millimeters, of the microscope dry objective and NA is a numerical aperture of the microscope dry objective on an entrance side, and

wherein the second lens unit is movable along an optical axis so that a fluctuation of aberration produced in accordance with variation of a thickness of a nearly plane-parallel plate interposed between an observation object and the first lens unit is corrected.

2. A microscope dry objective comprising, in order from an object side:

a first lens unit with positive refracting power, having two meniscus lenses, each with a concave surface facing the object side, and at least one positive lens;

a second lens unit with positive refracting power; and

a third lens unit having concave surfaces adjacent and opposite to each other configured as air contact surfaces,

wherein the following conditions are satisfied:

7≦ f

0.5<NA

where f is a focal length, in millimeters, of the microscope dry objective and NA is a numerical aperture of the microscope dry objective on an entrance side, and

wherein the second lens unit satisfies the following condition:

0.8≦|β2|≦1.2

where β2 is a lateral magnification of the second lens unit where an imaging point of the first lens unit is calculated as an object point.

3. A microscope dry objective comprising, in order from an object side:

a first lens unit with positive refracting power, having two meniscus lenses, each with a concave surface facing the object side, and at least one positive lens;

a second lens unit with positive refracting power; and

a third lens unit having concave surfaces adjacent and opposite to each other configured as air contact surfaces,

wherein the following conditions are satisfied:

7≦ f

0.5<NA

where f is a focal length, in millimeters, of the microscope dry objective and NA is a numerical aperture of the microscope dry objective on an entrance side, and

wherein a positive lens located on an image side of the two meniscus lenses of the first lens unit is movable along an optical axis so that a fluctuation of aberration produced in accordance with variations of ambient temperatures of the microscope dry objective and on a periphery of the microscope dry objective is corrected.

4. A microscope dry objective comprising, in order from an object side:

a first lens unit with positive refracting power, having two meniscus lenses, each with a concave surface facing the object side, and at least one positive lens;

a second lens unit with positive refracting power; and

a third lens unit having concave surfaces adjacent and opposite to each other configured as air contact surfaces,

wherein the following conditions are satisfied:

7≦ f

0.5<NA

60<D≦120

where f is a focal length, in millimeters, of the microscope dry objective and NA is a numerical aperture of the microscope dry objective on an entrance side, and where D is a distance in millimeters, measured along an optical axis, from an object surface to a nosepiece mounting surface of the microscope dry objective where a direction from the object side toward an image side is defined to be positive, the object surface refers to a surface in which an observation object lies and which is perpendicular to the optical axis of the microscope dry objective, and the nosepiece mounting surface of the microscope dry objective refers to an abutting surface where the microscope dry objective is mounted to a microscope body.