Ah! slowly sink Behind the western ridge, thou glorious Sun! Shine in the slant beams of the sinking orb, Ye purple heath-flowers! richlier burn, ye clouds! Live in the yellow light, ye distant groves! And kindle, thou blue Ocean!
So, this is British Romanticism: you're just going have to deal with a lot of "Oh!" and "Ah!", "Ye" and "Thou."
Now that he remembers the terrible tragedy that befell his friend Charles, the speaker orders nature to double its efforts to produce the greatest sunset ever. Come on, nature, we know you can do it. Do it for Charles!
He sees the sun "sink" on the horizon like an "orb" whose beams come at a "slant" rather than from directly above. The landscape is lit up by sunset, including the heath-flowers, the clouds, the "groves" of trees, and the ocean. Remember that this is only the speaker's imagination.
So my friend Struck with deep joy may stand, as I have stood, Silent with swimming sense; yea, gazing round On the wide landscape, gaze till all doth seem Less gross than bodily; and of such hues As veil the Almighty Spirit, when yet he makes Spirits perceive his presence.
These are the most difficult and philosophical lines in the poem. He wants Charles to be "struck" with happiness at the sudden sight of the sunset. He wants Charles to experience what he, the speaker, has experienced in the past. His senses will be "swimming," as if the air were filled with so many amazing visions that it became a different medium.
He will keep looking around until the whole landscape seems "less gross than bodily." "Gross" means "dense, thick, solid" (source), which is to say inhuman, which is how we normally think of nature. Rocks, for example, are dense, thick, and solid.
But after a while, the landscape will appear to Charles as if it had a "body" that contained a spirit, just as human bodies are thought to contain a soul. The landscape is merely a body or "veil" for the "Almighty Spirit" of God.
In other words, he can see the true nature of reality behind the superficial reality of appearances. For more on this trend in Coleridge's thought, see the "Calling Card" section.