A method for monitoring a chain pulley block and a chain pulley block apparatus with an electric actuator motor that is connected at the drive side to a transmission across a sliding clutch. The instrument and system are applicable to those specialties wherein the positioning accuracy of laser lesions is critical, such as whenever precise operations on a target or series of targets subject to movement during the procedure are to be effected. The optical system also comprises an optical concentrator element positioned substantially adjacent to each of the LED arrays, wherein each concentrator element reflects light emitted from the plurality of LEDs within the corresponding LED array so as to provide substantially uniform light at an output surface of each concentrator element. The sound that is produced can be disabled by rotating the whistle portion relative to the base portion or by urging a member so as to obstruct the flow of air through the whistle portion. Mounting methods are also provided. The stacked disks form a fluid flow path including at least two pressure reduction stages, a first pressure reduction stage being oriented so that the fluid flowpath is directed radially outward and a second pressure reduction stage being oriented so that the fluid flowpath is directed radially inward.