Answer: 

This type of situation comes up from time to time during Feng Shui consultations. When using a Convex mirror, the intention of use is to deflect back your neighbors negative energy that is being directed toward your home and self. 

In energy terms, this is a form of "non-battling." You are simply deflecting the energy they are projecting at you and your home to move around, up-over and away. The placement of the mirror depends on the location of your home to the neighbors home. It's suggested to place the mirror along the outside of your home (if possible). 

If need be, you could place the convex mirror along a window sill from the inside of your home. If you do this, make sure the reflective surface of the mirror is facing toward your neighbors home. Some consultants may disagree with this approach, yet if a person lives in an apartment building or for other practical reasons, this could be the next best method.

Very important, please read:

When you place the convex mirror for its intended use, you need to make sure you do not have any "ill-will" toward your neighbor. I know this may sound strange, yet again, I repeat, do not implement this cure while in a fowl mood. Be neutral and the effects on you when implementing this remedy is none. What you give out, is what you get back. The intention and envisioning of the convex mirror's use is to deflect. Think of it as Tai Chi for your home. 

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