Perspicacious, according to the Concise Oxford English Dictionary, is an adjective meaning "having a ready insight into and understanding of things." There are a few derivative forms of perspicacious. The adverb form of this word is perspicaciously, and the noun is perspicacity.
Perspicacity is a desirable quality in many professions, especially those that require a lot of troubleshooting. These fellows depicted below look mighty perspicacious. They seem to know exactly what needs to be done--just the kind of mechanics I would want working on my car.
Perspicuous is a related adjective, meaning "clearly expressed and easily understood; lucid." It has a second definition, referring to a person, meaning "expressing things clearly."
1963 Television Eyeglasses. From James Vaughn's photostream on Flickr.com. Some Rights reserved.
The television eyeglasses pictured below are perspicuous, but not because they help the guy in the photo see television better; they actually don't work. The gentleman below is inventor and futurist Hugo Gernsback. He is modeling a mock-up of a product he believed would be available soon--so the mock-up perspicuously demonstrates his concept.
1. First, in the 1960s, parents were always telling children not to sit too close to the television set, "or you'll go blind!" I can't imagine this item would have sold very well in those days, because people would have been afraid of it.
2. One might say Hugo Gernsback predicted Google Glass!