"Thirdly, Watch the full moon rising--how large and round she looks, resting as it were upon that eastern hill, and seen amidst the tops of its forest trees! How much larger and broader than when she hangs aloft in upper sky! Has every one here learned the true reason why? If not, look at her through a slit in a card, and her diameter will be the same. "Fourthly, A schoolboy is crossing his bedroom in the deep dark night, anxiously hoping that his head may not come into collision with the bed-post. Though carefully and successfully avoiding it, he imagines of a sudden that the blow is imminent. Quick as thought he stops to save his head, and, behold, the room is as quickly filled with sparks or flames of fire. Another moment, and all becomes dark once more. I have heard many a schoolboy exclaim over this phenomenon, but never knew one who could explain it. Finally, did you ever, on opening your eyes in a morning, close them quickly again, and keep them shut, directing them as if to look straight forwards? Most persons of active nervous power, after a few trials--say a dozen, or a score--are surprised to see colours appear and flit before the sight. Some years ago, Germany's greatest poet tried, at the suggestion of her greatest physiologist, a series of experiments on these coloured images. He found that by an effort of will he could cause them to come and go, govern their movement, march, and succession. And this took place under no conditions of impaired sensation, nor any hallucination of a diseased mind. A thoroughly healthy will succeeded in impressing itself upon physical instruments, controlling their law, and creating at its own pleasure an unfailingly bright phantasmagoria. "Some here may, others may not, have apprehended the distinctions between our five cases. The first two are due to the sensory apparatus, its optical laws of continued impression and complementary colour. In the latter three, mind intervenes. The enlarged size of the moon occurs through rapid comparison, the fiery lights in a dark room through instinctive apprehension, both influences of mind on the sensory system. The fifth and most interesting of all is no bad example of interference between moral and material law. The will truly causative (you may remark) overrules the natural process of physical impression, alters it, and creates a designed effect. I wish I could induce my young friends to devise a number of experiments on similar mixed cases, and, having tried them, to dissect out their real laws. These sharpenings of the critical faculty are exceedingly useful--they cultivate clearness; and most people know that two-thirds among our mistakes in life are caused by confusion of thought.