How can I make a mesh that I can see through?

Ok, I know that this isn't exactly a scripting question, but I've been having some trouble with this topic lately. In Call of Robloxia 5, the sniper scopes are simple cylinders, but I noticed that you can see through the sides, and when you use the scope, you can see directly through it. I've tried to imitate that by settings the Scale of a cylinder mesh to -1,-1,-1, but that never seemed to work. The only mesh that had some sort of an effect was the Head mesh, but you couldn't see through it when you looked directly at it. I asked this question because I've been trying to build a scope that didn't require many bricks but you could see through easily without it causing lag. Any suggestions?