An apparatus for illuminating and imaging the retina of an eye of a patient utilizes an LED as source of illumination. The processing applied to information to be transported to a public network, such as the Internet, is performed by the server rather than the modem. The temperature of the mass is influenced in a targeted manner in accordance with the opening width, and the friction losses in the mass that change as a result of this. The locking head also has an interlock member for releasably interlocking with the outer interlock engagement member of the strap body within the passageway.