Revell's Contacta Clear is a special crystal clear adhesive for model building.
It is a liquid-type adhesive that can be used for attaching transparent parts like windows, headlight lenses, etc.
When dry it has a high transparency and looks very clear, so that it doesn't show and it won't craze the clear parts.
With the brush in the bottle cap it is easy to apply a thin coat on both sides of the part you're working with.
One .71 oz bottle with integral brush, by Revell.