39．But in the heyday of portrait painting--fromthe late eighteenth century until the1850’s—anyonewith a modicum of artistic ability could become a limner assucha portraitist was called.

(as引导方式状语从句)

但是在肖像画的全盛期——从18世纪晚期一直到19世纪50年代——任何一个稍有艺术能力的人都能够成为一个绘画者(人们当时就是这样称呼一个肖像画家的)。

分句1：But in theheyday of portrait painting--from the late eighteenth century until the1850’s—anyone with a modicum of artistic ability could become a limner as

分句2：such a portraitist was called.

分句1是整个长句子的主句，中间的插入部分都是由介词引导，可见并不能算作分句

分句2是分句1的方式状语从句，由as引导

1和2嵌套