Theory:

When I returned from Lyonnesse
With magic in my eyes,
All marked with mute surmise
My radiance rare and fathomless,
When I returned from Lyonnesse
With magic in my eyes.
Explanation:
 
The first stanza reveals that the poet was having a lonesome life and set out on the journey to Layonnesse in pursuit of meaning to his life. The second stanza reveals how he is excited to know what the journey holds for him. Towards the third stanza, the readers can understand that, just like he mentioned in the second stanza, the journey did benefit him in a lot of ways. The poet, who started off as a hopeless person with a lot of burdens, returned from the journey with magic in his eyes and a crumpled piece of paper in which he had written this poem. When one is gloomy, one's eyes easily give him away. Similarly, when one gains new insights, it is reflected in one's eyes.
 
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Face with radiance
 
The poet does not reveal what exactly happened in the church. He does not explicitly state the happenings. But the readers can understand that it was something worthy and hope giving. It is also believed that Hardy met his wife Emma Gifford in Cornwall. The poet could also be referring to this incident as magic. He recalls that the people around could note that he was a changed person. They all marked with a mute surmise, indicting that they were taken aback and made assumptions as to what could be the cause of the sudden glow in his face. Usually, when people are happy, their face tends to glow. But he says that the kind of radiance that was seen in this face was a very rare one, the one that comes only when someone has completely attained spiritual satisfaction. He therefore says that the journey changed his life and brought meaning to his life.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1
SurmiseAssuming something as true without evidence
2
MuteNot speaking
3
RadianceGlowing happily
4
FathomlessUnable to be measured
Reference:
National Council of Educational Research and Training (2008). Honeydew. When I set out for Layonnesse - Thomas Hardy (pp. 105 - 106). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.