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Rh still occurs when the "N" rays traverse the reflecting surface from the inside towards the outside.

All these actions of "N" rays on light require an appreciable time-interval for appearing and disappearing. I was unable, although I varied the experiment in a great many ways, to observe any action of "N" rays on the refracted light.

I will here make the following general remark concerning the observation of "N" rays. The aptitude for catching small variations in luminous intensity is very different in different persons; some see from the outset, and without any difficulty, the reinforcing action produced by "N" rays on the brightness of a small luminous source; for others, these phenomena lie almost at the limit of what they are able to discern, and it is only after a certain amount of practice that they succeed in catching them easily, and in observing them with complete certainty. The smallness of the effects and the delicacy of their observation must not deter us from a study which puts us in possession of radiations hitherto unknown. I have