A method for teaching stringed instrument students how to play sheet music through the use of colored fingering numbers. The light-guiding member has a light reflection surface that reflects light to be plane-radiated from the LED and that allows the reflected light to be radiated in a desired direction, and a light radiation surface that allows direct light to be directly emitted from the LED and the reflected light to be externally radiated by the light radiation surface. The electron-beam system bombards each transistor with electrons that cause it to be selectively in a non-conductive condition in which its channel-region electric field is substantially insufficient for conduction or a conductive condition in which its channel-region electric field is sufficient for at least partial conduction. Any fluid can be tested, for example, urine or other body fluids, environmental samples, biological samples, clinical samples, or food and beverage samples.