Complex sentence: In the winter of 1889-90 he spent four months in the New York Postgraduate Medical School studying diseases of the eye, one of the first medical officers of the army to become expert in this field.
Split into simpler sentences: In the winter of 1889-90 he spent four months in the New York Postgraduate Medical School studying diseases of the eye. He was one of the first medical officers of the army to become expert in this field.