Question

The following passage is adapted from an essay on nineteenth-century optics and astronomy:

In the spring of 1800, the astronomer William Herschel conducted a series of experiments to measure the thermal energy associated with different colors of the visible solar spectrum. Passing sunlight through a glass prism, he projected the resulting rainbow spectrum onto a flat surface and placed sensitive glass-bulb thermometers within each colored band—violet, blue, green, yellow, orange, and red. To ensure that room temperature changes did not skew his measurements, Herschel positioned additional control thermometers in the shade near the spectrum table, away from the direct sunlight.

During his measurements, Herschel noticed a systematic increase in recorded temperature from the violet end toward the red end of the spectrum. Red light consistently registered a significantly higher temperature than green or violet light. Intrigued by this thermal progression, Herschel decided to measure the temperature just beyond the red edge of the visible spectrum, where no visible illumination was apparent. To his surprise, this dark region beyond the red boundary registered the highest temperature of all, proving the existence of invisible rays carrying heat energy—what Herschel initially termed "calorific rays" and what scientists later renamed infrared radiation.

In his papers presented to the Royal Society of London, Herschel carefully described his experimental apparatus. He mounted a glass prism on an adjustable stand positioned near a small opening in a window shutter, allowing a narrow beam of sunlight to enter the darkened room. The target surface holding the test thermometers was covered with thick white paper to render the boundaries of the spectral colors sharply visible. Herschel stressed that the shade-stationed control thermometers were crucial for verifying that ambient room air fluctuations were separate from the heat generated by the isolated light rays.

Based on the passage, where did Herschel place the control thermometers during his experiment?