Final answer:
Capsule staining is used to recognize a corneal ulcer by visualizing protective capsules in certain bacteria and yeasts. Negative staining techniques are used to achieve this, where the dye stains the background but not the capsules.
Step-by-step explanation:
Capsule staining is usually required for recognition of a corneal ulcer. Capsules are a protective outer structure found in certain bacteria and yeasts. These capsules do not absorb most basic dyes, so a negative staining technique is used to visualize them. In capsule staining, the dye stains the background but does not penetrate the capsules, making them appear as halos around the borders of the cells.