The effects of the illusion are extraordinary, but undoubtedly they are heightened by the imagination of observers, generally over-excited by fatigue, by privations, or sometimes by fever. These causes contribute to vary the nature of the phenomenon as seen by different eyes. Thus some gaze enraptured on verdurous islands bright as Armida's enchanted garden, with feathery palms and blooming flowers, and delicious sparkling lakes; others see, in that dim far-off which is never reached, the laughing waves of ocean, with ships resting calmly at anchor, or "Veering up and down, they know not why," and camels browsing quietly upon its shores; others, again, see before them the rolling river, its banks studded with groves and palaces; and all this, while there is not a solitary real object on the horizon whose presence might serve in some degree as a foundation for their visions. It is the very phantasmagoria of nature; her wildest, most wayward, and most fantastic sport. The reflection of the sky, modified by the inequalities of the soil and the vibratory movements of the air, can alone account for the singular deception. Imagination shows its victim, in the reflected image of the cloudless sky, a sheet of water, which is variously taken for a sea, a lake, or a river; it invests the slightest objects on the earth's surface with forms, colours, and dimensions, which are easily metamorphosed into houses, ships, men, animals; and it seems certain that those which in Nubia our fancy converts into camels would, in the Soudan, be transformed into elephants, and at Venice into gondolas. Imagination makes us its dupes, and gives to airy nothings "A local habitation and a name." It becomes absolutely necessary, therefore, to distinguish these wholly personal illusions born of a heated brain, from those which are really due to a definite physical cause. The latter necessarily suppose the existence of _actual objects_, below or very little above the horizon. Under such conditions, the most frequent illusion is that which shows the sky or rocks reflected in the expanse of rarified air superincumbent on the earth's surface, and which through this cause alone resembles water. It is then that the ignorant or inexperienced traveller, overwhelmed with fatigue and devoured by thirst, hastens his eager steps to reach more quickly that limpid water, where he hopes to refresh and reinvigorate himself, but which flies before his advance, and speedily vanishes altogether. Sometimes it is an inverted representation of terrestrial objects which appears in the air; or rather, these same objects, several times reflected, appear to multiply themselves. M. Trémaux relates that he saw the latter form of mirage in Nubia. He observed a row of doum-palms, which were about two thousand yards distant, repeated in several similar rows, each with a like number of trees, so as to produce the effect of a quincunx; among these trees floated several seeming sheets of water.