A day or two later Mrs. Strickland sent me round a note asking if I could go and see her that evening after dinner. I found her alone. Her black dress, simple to austerity, suggested her bereaved condition, and I was innocently astonished that notwithstanding a real emotion she was able to dress the part she had to play according to her notions of seemliness.

simple to austerity，是形容词短语，作dress的非限制性定语。相当于：Her black dress, which was simple to austerity, suggested her bereaved condition。

to austerity 是simple的结果状语，说明“简单到朴素”，指穿着很简朴。