seeing the invisible

It is known to us all that people usually apply visual methods to keep secrets. For example, in the story " The Poisoned Pen", the murderer uses invisible inks to implement crime. It is really hard for people to detect such invisible evidence.

However, spies need to build such skills and use technology to make their vision keener. There are various kinds of tools help them see the invisible.

Magnifying lenses, which can zoom in, give people a closer view of private scenes. In additon, some photograph subjects in near darkness can still work. They carry with infrared light source, or night-vision optics, producing better photographys of documents. Such equipment gives spies abilities to see in very dim light. Also, they use the equipment looks like today's binoculars using angled mirror to see special scene.

It remind me of Detective Dee. I saw this movie yesterday night. Becuase of the struggle for power in Palace, some people use poison dissolved in water to kill people. I think such measure also belongs to invisible method because water will not attract people's attention and will not arise people's suspicion.