Which refers to the entire previous sentence "They flash upon that inward eye".
Inward eye means "mind", and "bliss" means "blessing and comfort".
“They flash upon that inward eye, Which is the bliss of solitude;” translated as “This scene flashes in my mind, comforting my loneliness many times;” or “This scene is haunting my dream. Holding hands, lingering loneliness."
The entire poem is translated as follows
I am like a lonely flowing cloud,
Floating high above the valley,
Suddenly I saw a large field of flowers,
golden narcissus blooming everywhere.
They bloom by the lake and under the trees.
They dance and drift in the wind.
They are as dense as the stars in the Milky Way,
Like stars shining in a crystal;
They stretch forward along the bay,
The way to the distance seems endless;
At a glance, there are thousands of flowers,
How happily the thousands of flowers shake their heads and dance.
The sparkling lake waves danced happily nearby,
but not as gracefully as the narcissus;
The poet met this happy traveling companion,
How can I not feel joyful and excited;
I stared at it for a long time - but did not realize it
The spiritual treasure this scene gave me.
How many times later did I lie alone in depression,
I felt bored and empty;
This scene flashed in my mind,
How many times It comforted my loneliness for the first time;
My heart danced with the narcissus again,
My heart was filled with joy again
Guo Moruo translated as follows:
In the mountains and valleys, white clouds floated
I was like a white cloud, traveling alone
Suddenly I saw narcissus, and the yellow flowers were quiet and quiet
Under the trees by the lake , dancing endlessly
Like the Milky Way, shining with stars
Narcissus stretches as far as the eye can see
Thousands of flowers, refreshing to the eyes
In the wind Swinging, lively and joyful
The lake in the valley, the blue waves are rippling
The flowers dance in the breeze, joyfully beating the waves
Indulge in it and stare at it for a long time
What a treasure without realizing it
Sometimes I lie down, confused and sad
Daffodils by the lake leap into my heart
Sudden enlightenment, the blessing of loneliness
< p>My heart is beating and I dance with the flowersThe original author of the poem is William Wordsworth (1770～1850), a British Romantic poet.