What is materialization spiritually?

If the spheres exist around the earth, why it is that all men cannot see them, and why cannot they even see those spirits who are said to be actually in the room with them?

If you see photographs of unmaterialized spirits, you will notice that to your material sight they present a semi-transparent appearance. 

The material doors and windows, furniture, and so on, show through the figures of the spirits.

The material particles of an astral body―or first degree of spiritualized matter are spread so thinly that they are like a fine network united by invisible atoms of a more etherealized nature, so sublimated, that spirits must either materialize an earthly body or make use of astral envelopes or bodies from clouds of semi-material human atoms. 

Spirits can mould the astral shells into their own likeness, as a sculptor moulds clay.

Spirits make a body from the more material atoms and clothe themselves in it. These replicas can be photographed and bear a resemblance to the spirit―each spirit stamps his own identity on the plastic astral form or prepares that form to receive his impression.

The spiritual spheres and their inhabitants are more transparent and intangible to material sight, yet they exist and are solid. 

They are only invisible to our imperfect sight, which is limited to material things of comparatively great density. 

Our spiritual sight is closed and we can only see what is material.