During the study of the action of the ultraviolet light on the blood of the new-born infant, it was found that there was an increase in the white blood cells and a change in their morphology.1 It was therefore considered desirable to study this action in detail.
This study is based on a series of 100 new-born infants, during a period of ten months. The differential counts were made within six hours after birth and at twenty-four hour intervals thereafter. The determinations were made at the same time every day. The slides were stained with carbol-pyronin,2 the films being dried in air and the stain applied for fifteen minutes. No preliminary fixation is required after washing and drying. The slides are examined unmounted. The stain imparts a deep blue color to the nuclei and a pink color to the cytoplasm. The red cells stain feebly pink.In