temperature - Why does William Herschel's experiment show red light as warmer than blue?

Why does William Herschel's experiment show red light as warmer than blue if blue light is higher energy?

Here is an explanation of Herschel's experiment http://www.ipac.caltech.edu/outreach/Edu/Herschel/backyard.html

In short, Herschel placed multiple thermometers on the light separated by a prism. This led to the discovery of infrared light as he placed his control thermometer outside of the visible spectrum on the red side.

You may see similar apparatus to his here:

As you can see, the red thermometer is reading a higher temperature.

My guess is that blue is scattered more, therefor the thermometer for red absorbs more light.