BACKGROUND OF THE INVENTION  1. Field of the Invention
 This invention relates to multiphoton-excitation observation apparatuses.
 This application is based on Japanese Patent Application No. 2006-088760, the content of which is incorporated herein by reference.  2. Description of Related Art  A multiphoton-excitation apparatus is a known apparatus in the related art for observing the function of cells and so forth by irradiating a biological specimen or the like with ultrashort pulsed laser light from the surface thereof and detecting the fluorescence emitted from the surface or the interior of the specimen due to multiphoton absorption (for example, see Publication of Japanese Patent No. 2848952).
 Also known in the related art are scanning laser microscopes which acquire image data in a two-dimensional XY plane by detecting reflected light or transmitted light from a specimen while scanning a spot of light in the two-dimensional XY plane on the specimen, and which acquire image data at different depths in the specimen, ranging from a surface layer portion to a deep layer portion, by varying the focal position of the spot of light in the Z direction.
 However, when the focal position is varied in the depth direction in this way, there is a phenomenon whereby the brightness decreases the deeper each acquired image becomes in the Z-direction. Conventionally, therefore, a technique for correcting the apparent brightness by subjecting the light level of the image data acquired in the observation to post-processing to obtain easily observed images with uniform brightness has been proposed (for example, see Japanese Unexamined Patent Application, Publication No. 2000-275541).
 As biological research has progressed in recent years, there have been increasing demands for observing regions deeper inside biological specimens. A standard single-photon-excitation laser scanning microscope can acquire an optical internal tomogram of a specimen under observation by virtue of the confocal effect; however, the observable depth in the biological specimen is only about 50 um from the surface.
 A multiphoton excitation method described in Publication of Japanese Patent No. 2848952 excites fluorescence at near-infrared wavelengths, which have high transmittance through biological tissue and, therefore, are suitable for observing deep regions. However, even with multiphoton excitation, when observing a deep region exceeding a depth of 100 um, for example, the effects of scattering, refraction, and absorption of the excitation light and the fluorescence by the specimen are strong, resulting in the problem that the resolution and S/N ratio decrease, which makes it impossible to acquire clear images.
BRIEF SUMMARY OF THE INVENTION
 An object of the present invention is to provide a multiphoton-excitation observation apparatus which can acquire clear images from the surface layer portion to the
deep layer portion, even when observing down to a very deep region (for example, to a depth of about 100 um or more from the surface).
 The present invention provides a multiphoton-excitation observation apparatus comprising a light-source unit; an observation apparatus main unit for irradiating a specimen with laser light emitted from the light-source unit and observing fluorescence emitted from the specimen; and a control apparatus. The light-source unit includes a pulsed laser light source for emitting pulsed laser light; and an incidence-adjusting unit for adjusting the beam diameter of the laser light emitted from the pulsed laser light source. The control apparatus controls the incidence-adjusting unit according to the depth of an observation plane in the specimen.
 With this configuration, laser light with a beam diameter that is suitably adjusted by the control apparatus according to the depth of the observation plane is radiated. The inventors of the present application have confirmed experimentally that, as the observation depth increases, the clarity of the observation image of the deep layer portion is improved by stopping down the beam diameter of the radiated laser light to smaller than the original beam diameter of the objective lens for achieving the designed performance. Thus, by varying the radiated beam diameter according to the observation depth, it is possible to acquire an observation image having good resolution at any depth, and it is possible to acquire clear fluorescence images from the surface layer portion to the deep layer portion.  In the multiphoton-excitation observation apparatus described above, the control apparatus may include a memory unit for storing, in association with each other, observation plane depths in the specimen and operating levels of the incidence-adjusting unit; may obtain the operating level corresponding to the depth of the observation plane from the memory unit; and may control the incidenceadjusting unit with the obtained operating level.  Thus, by providing the memory unit for storing the observation plane depths and the optimum operating levels in association with each other, it is possible to quickly control the incidence-adjusting unit with an operating level that corresponds to the observation depth. Accordingly, it is possible to adjust the beam diameter of the laser light to the optimum diameter for the depth of the observation plane.  The information stored in the memory unit is obtained as described below, for example.  For instance, by adjusting the incidence-adjusting unit in a predetermined operating range with at least two reference depths selected in an observation range related to the depth direction of the specimen and with the observation plane set at a reference depth, the beam diameter of the laser light is varied and the operating level of the incidenceadjusting unit that is optimal for each reference depth is specified from the observation image acquired at this time. Then, the optimum operating level for each observation plane depth is determined based on the relationship between the reference depth and the operating level of the incidenceadjusting device and is stored in the memory unit.  In the multiphoton-excitation observation apparatus described above, the control apparatus may adjust the intensity of the laser light irradiating the specimen according to the depth of the observation plane in the specimen.  With this configuration, the pulsed laser light emitted from the light-source unit is supplied to the observation