An optical system for detection of atmospheric ice particles is described. The image forming apparatus compares the user information with the access information and outputs an image only when the output user conforms to the particular person. The frame is provided with an open space at both eyes between the mica and the end of the rim, which provides ventilation and prevents the fogging of the graduated micas. A ball unit holds a free ball which rolls in an exposed manner on both a surface and a back surface of the chain link. Components of the communication systems use the control signals to automatically select a denoising method appropriate to data of frequency subbands of the acoustic signals. The connecting device includes a spacer having a thickness substantially equal to a prescribed cell gap, and an operating device for inserting the spacer in between the substrates, or withdrawing the spacer from the same.