Interpupilary adjustment – B
The distance between the pupils of each eye isn't the same for everyone and this adjustment compensates for that. As you look down the eye pieces hold an eye piece in each hand and spread them apart or push them together until the two images are super imposed. When this happens you'll sense the depth of field of the binocular view of the specimen. The small dot gives you the distance between your pupils - make a note of the number so it can be set quickly the next time you use the microscope.