Natyashastra
मुखरागान्वितो यस्मान्नाट्यमत्र प्रतिष्ठितम् । यथा नेत्रं प्रसर्पेत मुखभूदृष्टिसंयुतम् ॥ १६९॥
Since dramatic representation is established through facial expression, the movement of the eyes should be accompanied by the appropriate color of the face and glance.
english translation
mukharAgAnvito yasmAnnATyamatra pratiSThitam । yathA netraM prasarpeta mukhabhUdRSTisaMyutam ॥ 169॥
hk transliteration by Sanscript