Nuclear sclerosis increases the refractivity of the ocular crystalline lens as animals get older.
This increased refractivity causes the two pinpoint lens reflections seen in the eyes of young adult dogs and cats to appear to increase in size. Increasing refractivity also causes the clear lens nucleus to develop a faint blue-gray appearance that becomes increasingly intense. When categorized, these changes can be used to divide life span into age groups, thereby allowing the age of mature dogs and cats with unknown histories to be determined more accurately than is possible using the dental method of estimating ages.