A thermal component of excitation in the lateral eye of Limulus.

1. The temperature dependence of the relative spectral sensitivity of the excised lateral eye of Limulus was examined using its electrical response to light.2. At wave-lengths longer than 625 mmu lowering the temperature from 27 to 7 degrees C reduced the relative spectral sensitivity, while no effect was measurable at shorter wave-lengths.3. The reduction in relative sensitivity increased linearly with decreasing wave number.4. The observations support the hypothesis that a critical amount of energy (activation energy) must be supplied to the photopigment molecule in order that it excite the photoreceptor. Quanta with energy lower than the activation energy are effective only if the thermal energy of the molecule can supply the deficit.5. The lower limit of the activation energy for the photoreceptor of the lateral eye of Limulus determined from the results of this study is 44 kcal mole(-1).