Charging problem '95 petrol model

Greetings. My '95 323 is suffering a charging problem. If I charge the battery up fully, the car will run nearly 45-50 miles in the dark before showing symptoms of trouble, at which point the lights begin to dim, the charging warning light suddenly comes on, and the engine dies. From all this, I assume it is a case of the alternator's rate of output, rather than a complete lack of charge. I cannot hear or see any signs of belt slippage when the engine is running, however. What should the correct tension be, anyway?

Any other suggestions?