If our eyes only saw frames (IE: 30 images a second), like a single lens reflex camera, we'd see images pop in and out of existance and that would really be annoying and not as advantagous to us in our three dimensional space and bodies.
In the Vienna of the second half of the 18th century, in addition to Haydn and Mozart, three symphonic composers, namely Leopold Hoffmann, Carl Ditters von Dittersdorf and J.B. Vanhal helped further the development of the symphony.
The earliest symphony with trombones in the orchestra was apparently one by Joseph Krottendorfer in 1768. There were a very few other eighteenth-century symphonies with trombone parts, notably three symphonies in the French style that Ignaz Pleyel composed for his visit to London as Haydn's rival in 1792.