Lenard began the study of cathode rays in 1888. These rays were made in simple, partially evacuated glass tubes. Inside were metallic electrodes across which a high voltage could be placed. It was difficult to study the rays inside the sealed glass tubes. They were difficult to access, and and there were still air molecules in the tubes. Lenard was able to make small metallic windows in the glass, thick enough to be able to withstand the pressure, but thin enough to let the rays pass through. He could pass the rays out into the laboratory, or into another chamber that was completely airless. These windows are now known as "Lenard windows". He could easily detect the rays and measure their intensity by using paper sheets with a layer of phosphorescent materials.