Verse 1.7.4

अथ यदेतदक्ष्णः शुक्लं भाः सैवर्गथ यन्नीलं परः कृष्णं तत्साम तदेतदेतस्यामृच्यध्यूढंसाम तस्मादृच्यध्यूढंसाम गीयते । अथ यदेवैतदक्ष्णः शुक्लं भाः सैव साथ यन्नीलं परः कृष्णं तदमस्तत्साम ॥ १.७.४ ॥

atha yadetadakṣṇaḥ śuklaṃ bhāḥ saivargatha yannīlaṃ paraḥ kṛṣṇaṃ tatsāma tadetadetasyāmṛcyadhyūḍhaṃsāma tasmādṛcyadhyūḍhaṃsāma gīyate | atha yadevaitadakṣṇaḥ śuklaṃ bhāḥ saiva sātha yannīlaṃ paraḥ kṛṣṇaṃ tadamastatsāma || 1.7.4 ||

4. Further, there is a white glow in the eyes, and this is compared with the Ṛg Veda. Then there is a similar glow which is a deep, dark blue. This is compared to the Sāma Veda. This dark glow is based on the white glow. This is why the Sāma is said to be based on the Ṛk. Sā of Sāma stands for the white glow in the eye, and ama stands for the deep blue glow. These two together are Sāma.

Word-for-word explanation:

Atha, then; akṣṇaḥ yat etat śuklam bhaḥ, that which is the white glow in the eyes; saḥ eva ṛk, that is the Ṛg Veda; atha yat nīlam paraḥ kṛṣṇam, then that which is blue and very dark; tat sāma, that is the Sāma; tat etat sāma, this Sāma; etasyām ṛci adhyūḍham, is based on the Ṛk; tasmāt, this is why; ṛci adhyūḍham sāma gīyate, the Sāma is sung as based on the Ṛk; atha yat eva etat akṣṇaḥ śuklam bhāḥ, then this white glow in the eye; sā eva sā, that is sā; atha yat nīlam paraḥ kṛṣṇam, and that which is blue and very dark; tat amaḥ, that is ama; tat sāma, that [if joined together] is Sāma.

Commentary:

The sun is the support of the power of vision, because without the sun we cannot see. Similarly, the deep blue glow, which is identified with the Sāma, is supported by the white glow, which is the Ṛk.