Abstract: A method of using a collection apparatus as well as the collection apparatus itself. The latter includes collection receptacles, a drip nozzle, and a component for bringing about a relative movement between the receptacles and the drip nozzle at predetermined times to collect the drops emitted by the drip nozzle, in the different receptacles. The time between the drops is measured by a time measurement circuit. The lower and the upper time limits for a time interval within which the next drop is expected, are calculated by a time limit calculating circuit. A blocking circuit is adapted to compare the time measured after the latest drop with the calculated time limits, and to block the relative movement when the measured time lies between these time limits. The blocking circuit is adapted to permit the relative movement when the expected drop has been emitted within the time limits.

Abstract: A device for keeping an outer surface of an optical element clean, said element being located in surroundings where deposits can be quickly formed on said element. The device has a protective device or screen with an opening placed in front of the surface to be protected. This opening has inner walls extending out from said surface of the optical element. The screen has a slot, which extends peripherally around the edge of the opening facing the surface to be protected, and gas is forced to stream through the slot. At least a part of the inner surface of the opening has an inwardly projecting part having a surface remote from the optical element which is parallel to or inclined outwardly from a symmetrical plane through the element.