It is not sensible to vision.

It makes an impression upon our feelings.

It acts powerfully upon all bodies.

It has no weight.

It attends, or is connected with, all the operations of nature.

It radiates from all bodies in straight lines, and in all directions.

It strikes most powerfully in direct lines.

Its rays may be collected into a focus, just as the rays of the sun.

It may be reflected from a polished surface.

It is more easily conducted by some substances than by others.