Period instruments
The interest in period instruments started in the mid 20th century.  People like Wanda Landowska played Baroque keyboard music on the harpsichord instead of the piano.  Conductors like Nikolas Harnoncourt have trained small orchestras to play on period instruments using techniques which were used in Baroque times.  Gradually musicians also started using period instruments for music from the Classical music period (the time of Mozart and Beethoven).  Even in the mid 18th century the modern orchestra had not yet evolved.  In recent years conductors like John Elliot Gardiner have performed music by Hector Berlioz on period instruments.  There are now many small instrumental groups and orchestras who regularly perform on period instruments e.g. The English Concert and the English Baroque Soloists.