One doesn’t often encounter all-Mendelssohn programs. If I were to find one in the Tanglewood season, I’d suspect it was a somewhat excessive gesture towards the more conservative members of the audience. On the other hand, from the Scottish Chamber Orchestra and Frans Brüggen, who has maintained a long-term relationship with the orchestra over the years, it meant a fresh look at three important works by a towering figure in nineteenth century music. Our view of Mendelssohn is still colored by the popular conception of him as a genial, highly privileged composer of tuneful works, who sadly died at the young age of thirty-eight. In truth, he was, both as a composer and a conductor, an extremely influential leader in the highly theoretical and factionalized world of Romantic music, the central figure in the more conservative, “classizing” group based in Leipzig.