The Vision of Preliminary Meditative Concentration

When the mind settles down while observing the breath, with eyes open one feels a patch of darkness before one's eyes—this is a good sign of preliminary meditative concentration (samādhi). Previously the mind chased after the objects of the six sense objects; now the mind turns inward and no longer sees the sense objects, yet it is still veiled by the domain of the aggregate of form, so everything before the eyes appears dark. If one continues to deepen one's meditative concentration, even with eyes closed one will feel brightness before the eyes, but there is still a long way to go before breaking through the domain of the aggregate of form.