The internal structure, also, was still very uncertain. The first movement, now usually written in strict sonata form, did not then uniformly contain the two contrasting themes, nor the codas and episodes of the modern schools; and the working-out section and recapitulation were seldom clearly defined. Even in the poorest examples, however, the sonata scheme was generally vaguely present; and in the best often definitely marked. We must not lose sight, however, of the epoch-making work of Stamitz and his associates at Mannheim, both in the fixing of symphonic form and the advancement of instrumental technique. Stamitz’s Opus I appeared, it will be recalled, in 1751; Dittersdorf’s emulation of the Mannheim symphonies began about 1761. The intervening decade was a period of experiment and constant improvement. Haydn, though his first symphony, composed in 1759, showed none of the new influence, must have been cognizant of the advance. Haydn’s first symphony, written when he was twenty-seven, is described by Pohl as being a ‘small work in three movements, for two violins, viola, bass, two hautboys, and two horns; cheerful and unpretending in character.’ From this time on his experiments in the symphonic form were continuous, and more than one hundred examples are credited to him. He was so situated as to be able to test his work by actual performance. To this fortunate circumstance may be attributed the fact that he made great improvements in orchestration, and that he gained steadily in clearness of outline, variety of treatment, and enlargement of ideas.