Remote SO2 measurements using COSPEC

A teaching machine apparatus is provided for teaching symbols such as numerals or letters along with their interrelationship, having a flashing light source in a casing. The casing has a segmented front having each segment having a transparent portion to allow the light from the light source to pass therethrough. Means are provided on the casing for attaching coded members over the transparent portions of the casing which coded members have openings located in predetermined locations. A symbol bearing member can be placed over each coded member which member also has a predetermined opening therein. When the correct symbol bearing member is placed over the correct coded member their opening will align, allowing the passage of light therethrough. The unit also provides for a light reflector so that a lamp can be used for a plurality of transparent openings and also for the transparent openings to be covered with transparent colored material for providing different colored flashing lights.