 His contributions in the field of pathology included improving techniques and standardisation of tissue staining; his book, written with James Homer Wright, was the standard textbook in this field.He also studied the function of histiocytes, he confirmed that the whooping cough bacillus discovered by Jules Bordet was the causative agent, and he worked on improvements in classification of tumours, particularly meningiomas, and cirrhosis of the liver.He was president of the American Association of Pathologists and Bacteriologists in 1910, and was its treasurer from 1911 to 1940.