A method for operating an internal combustion engine having at least two exhaust-gas turbochargers, which allows the synchronous operation of the two exhaust-gas turbochargers to be reliably set. The optical data recordingreproducing method includes a first trial writing operation in which trial writing data are recorded into the recording medium while changing the recording power conditions, the recorded trial writing data are reproduced, and the reproduced trial writing data are evaluated to set an optimum recording power. This allows correct determination of the number of recording layers of recording medium while not being affected by variation in property of the instruments, variation in reflectivity of recording medium, variation in the ambient temperature and fake pulse corresponded to waviness in focus error signal. A holographic unit has a first hologram suited to the first laser diode and a second hologram suited to the second laser diode, the first hologram provided to diffract a reflection beam of the laser beam of the first laser diode to the photodetector unit, and the second hologram provided to diffract a reflection beam of the laser beam of the second laser diode to the photodetector unit. A valve is configured to control a rate of fluid delivered from the source to the site via the tube, and a remote control unit is configured to enable a user to control the valve to adjust the rate of fluid delivered from the source to the site.