In the beginning this term was used to describe the strange white substance that came from the body of the medium during seances. Decriptions are of all shapes and sizes but it seems to look like a gelantinous, mostly white jelly like substance that would flow from different parts of their bodies mostly their mouth. Many believers at the time thought this was actual manifestation of an astral body. Mediums claimed that this substance caused them pain when exposed to light or touched. Now whether or not you believe this is entirely up to you..
If science could disprove alot of the evidence from investigations such as Orbs, Ectoplasm, and strange paranormal activity then Im sure they would. Whatever you may believe in: the validity of Ectoplasm or not, the evidence is apparent, countless pictures exist that cannot be explained naturally but it is very important that all pictures be scrutinized thoughoughly before any official "Ghost Sighting" is rendered.
As with taking pictures, using EMF meters, video tapes and the like, extreme care should be given to any serious investigation. It would be great to get actual ghost photo's or apparition's on tape but we all know it isn't that easy. Always be aware of your surroundings and the weather. In pictures, fog, gas, smoking cigarettes, campfires, steam, glare, reflections from something else, smoke, condensation, "methane", rain, snow, sleet, etc, etc. can all be contributing factors in any and ALL photographs. If you cannot rule out all of these natural occurances then chances are your picture will be discredited even if you feel it was real or not.
