Psalm 19:8: Instructions and Commandments
The instructions of ʏʜᴡʜ are just, making glad the heart. The commandments of ʏʜᴡʜ are pure, brightening the eyes. (Psalm 19:8 [Heb. verse 9])
The word "just" here translates יְשָׁרִים (yᵉshā-rịm). When used literally, the word refers to that which is "straight", "level", or "smooth", for example, a smooth path. In Scripture, justness is contrasted with iniquity (for example, Deuteronomy 32:4) which is a mistreatment in your dealings with others, and with sin (for example, Psalm 25:8) which is a "missing the mark" — a moral failure. There is nothing crooked or deviant in the instructions of ʏʜᴡʜ, because there are no shortcomings or peversity in the motivations or behavior of ʏʜᴡʜ himself. Those who embrace and practice ʏʜᴡʜ's instructions will become glad of heart. The Hebrew word מְשַׂמֵּחַ (mᵉsham-mē-ḥa) means "to gladden" or "to make merry".
The commandments of ʏʜᴡʜ are pure. "Pure" translates the Hebrew word בָּרָה (bā-rā) which is a form of the adjective בֵּר (bēr). This word is only used a few times in Scripture, but usually it describes those who are pure in heart, and therefore suitable to be in God's presence or to receive his blessing.
Who shall ascend the hill of the Lᴏʀᴅ? And who shall stand in his holy place? He who has clean hands and a pure heart, who does not lift up his soul to what is false and does not swear deceitfully. He will receive blessing from the Lᴏʀᴅ and righteousness from the God of his salvation. Such is the generation of those who seek him, who seek the face of the God of Jacob. (Psalm 24:3-6)
Other rules for living, derived from satanic or mere human thinking, will be tainted by blasphemy or moral depravity. But the commandments of ʏʜᴡʜ are pure, suitable for those who desire to be in the presence of ʏʜᴡʜ. I have translated the phrase מְאִירַת עֵינָיִם (mᵉʾē-rat ʿē-nā-yim) as "brightening the eyes". Leupold understands this as giving a "freshness and joy" that affects even "the looks of the eye".
 See HALOT.
 Leupold's understanding seems to be in line with the context here and the other uses of the phrase in Scripture. In Proverbs 29:13, it is said that "the Lᴏʀᴅ gives light to the eyes of both" the poor man and the oppressor (ESV) which seems to emphasize that they both receive their vitality from God. In Ezra 9:8, God is said to be "brightening our eyes" (ESV) as he gives "favor" and "reviving" to the slaves of the captivity.