In Rachel Carson's book Silent Spring, she brings the issue of pesticide use to the attention of the public, the scientists and the industries associated with pesticides. In her book she offers irrefutable evidence of the harm pesticides had already done, but also offers ways to correct the damage. In the metaphor, Carson says the public is given half truths which means that the words have enough truth to sound good but not really give the public enough information to know what is happening. Tranquilizing pills keep a person or the public of a nation doped up enough to not be upset about anything--even the negative prospects of pesticides and the damage they have already done. So half-truths get wrapped up inside tranquilizing pills which keep the public happy and not digging for the real truth. This metaphor is so effective because it is exactly the right description for what has been happening and how the public has been kept quiet, uninformed and duped into ignorance of the real truths. Carson is tearing away the veil of concealment which half-truths had provided until her book appears.