Whose glory did Isaiah see?

Isaiah 6:1-5 NWT

In the year that King Uz·ziʹah died, I saw Jehovah sitting on a lofty and elevated throne, and the skirts of his robe filled the temple. Seraphs were standing above him; each had six wings. Each covered his face with two and covered his feet with two, and each of them would fly about with two.
And one called to the other:
“Holy, holy, holy is Jehovah of armies.
The whole earth is filled with his glory.”
And the pivots of the thresholds quivered at the sound of the shouting, and the house was filled with smoke.
Then I said: “Woe to me!
I am as good as dead,
For I am a man of unclean lips,
And I live among a people of unclean lips;
For my eyes have seen the King, Jehovah of armies himself!

Who did Isaiah see?
John 12:36-41 NWT

While you have the light, exercise faith in the light, so that you may become sons of light.” Jesus spoke these things and went off and hid from them. Although he had performed so many signs before them, they were not putting faith in him, so that the word of Isaiah the prophet might be fulfilled, which he said: ‘Jehovah, who has put faith in the thing heard from us, and as for the arm of Jehovah, to whom has it been revealed?‘ The reason why they were not able to believe is that again Isaiah said: ‘He has blinded their eyes and has made their hearts hard, so that they would not see with their eyes and understand with their hearts and turn around and I heal them.’ Isaiah said these things because he saw his glory, and he spoke about him.

Whose glory did Isaiah see?
Who is "the arm of Jehovah"?
John 1:18 NWT

No man hath seen God at any time; the only begotten Son, which is in the bosom of the Father, he hath declared him.

If nobody has seen God at any time... who did Isaiah see when he saw Jehovah?