How do spirit-lives materialize?

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 unmaterialised 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 spiritualised matter are spread so thinly that they are like a fine network united by invisible atoms of a more etherealised nature―so sublimated that spirit-lives must either materialise an earthly body―or make use of astral envelopes―or bodies from clouds of semi-material human atoms. 

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

 

Spirit-lives 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 your imperfect sight, which is limited to material things of comparatively great density.  

Your spiritual sight is closed and you can only see what is material.