The anatomical nucleus of the human lens can already be isolated at an age of 25 years; its size increases with age. The resistance to penetration of the normal human lens increases with age and although this variation is not very important, it may play a secondary part in the genesis of presbyopia. In all forms of senile cataract with a pigmented nucleus, the resistance to penetration is noticeably increased. This phenomenon is particularly pronounced in black cataract. The word ‘sclerosis’ of the lens should be eliminated from the scientific vocabulary.