 To create nucleopore filters, the technique is applied to a polycarbonate film and the etching is allowed to continue from both sides of a sheet of plastic until the two holes are connected, resulting in a tiny hole in the sheet.Continuous dissolving thereafter slowly and predictably widens the hole diameter until the desired diameter is obtained.Use of the technique in an experiment carried by a high-altitude balloon in 1975 resulted in the detection of one highly anomalous cosmic ray particle that traversed a stack of 32 sheets of Lexan plastic.