Vaisesika Sūtra 4.1.12

अरूपिष्वचाक्षुषाणि ॥१२॥

arūpiṣv acākṣuṣāṇi ॥12॥

[अरूपिषु (arūpiṣu) - in formless entities; अचाक्षुषाणि (acākṣuṣāṇi) - are not visible;]

The formless entities are not visible.