A blade structure for centrifugal fan includes a hub, a first blade unit, and a second blade unit. The display also includes a porous dielectric layer formed on the substrate and the columns. The emitter is at least partially encased within the housing. In one embodiment, the disk drive has means for communicating with a controlling host computer which implements the HCC protocol and a user computer which does not implement the HCC protocol, to allow the user computer to be protected from HCC in a transparent way. The tactile control module controls the frictional force between the insulator and the user by outputting to the conductor an electrical signal regulated by the parameters determined by the processing unit. The bias forces are applied so as to press against a front surface of the mirror in the vicinities of the two fixed points and the adjustable point, which tilts the mirror, so that the back surface of the mirror touches the two fixed points and the adjustable point.