The other day I changed a light bulb, and then stood there for a minute holding the old one. The bulb I had taken out was an incandescent: sixty watts, around eight hundred and sixty lumens, rated for about a thousand hours. The one going in draws nine and a half watts for the same light and is rated for twenty-five thousand. Same socket, same room, same brightness. A sixth of the power, twenty-five times the life, a few dollars at the hardware store. That is an absurd bargain, and yet it has become nearly invisible. A light bulb is...