Patent ID: 7791795

Claim:
A microscope provided with a focusing assist apparatus, comprising: a light source; a first lens group; a diameter-variable diaphragm which is disposed at a position which is conjugate with said light source while said first lens group is interposed therebetween; an optical path branching member which is illuminated with a luminous flux restricted by said diameter-variable diaphragm, said optical path branching member having a focusing indicator and forming two luminous fluxes, the luminous fluxes being inclined by predetermined angles from an optical axis; a second lens group; and an objective lens, wherein said diameter-variable diaphragm is disposed at a position which is conjugate with a pupil plane of said objective lens while said second lens group is interposed therebetween, said focusing indicator of said optical path branching member is disposed at a position which is conjugate with an observed surface by said second lens group and said objective lens, said optical path branching member is insertable into and is retractable from a microscope optical path, a diameter φ of said diameter-variable diaphragm is set at a predetermined value suitable to focusing assist when said optical path branching member is inserted into the microscope optical path, and the setting suitable to the focusing assist is given by satisfying simultaneously the following conditional expressions (1) and (2), [ Formula ⁢ ⁢ 1 ] β 1 ⁢ a ≥ ϕ M ( 1 ) 2 ⁢ L ⁢ ⁢ tan ⁡ ( ɛ - tan - 1 ⁢ ϕ 0 2 ⁢ L ⁢ ⁢ β 2 ) < ϕ < 2 ⁢ L ⁢ ⁢ tan ⁡ ( ɛ + tan - 1 ⁢ ϕ 0 2 ⁢ L ⁢ ⁢ β 2 ) ( 2 ) where β 1 is magnification of an image formed at a position of said diameter-variable diaphragm by said light source through said first lens group, a is a size of said light source, φ M is a maximum diameter of said diameter-variable diaphragm, ε is an inclination of a luminous flux given by said optical path branching member, L is a distance between said diameter-variable diaphragm and said focusing indicator, φ 0 is a pupil diameter of said objective lens, and β 2 is magnification of an image of said diameter-variable diaphragm formed at a position of the pupil plane of said objective lens through said second lens group.