In taking a binocular picture of a machine or instrument consisting of various parts, that minute accuracy which is necessary to give the true form and expression of the human face is not required; but if it should happen that, in a correct binocular view of the object, parts are concealed which it would be useful to see, we must discover the binocular angle which will shew these parts in the two pictures, or, generally speaking, which will give the best view of the mechanism, and then adjust the lenses of the camera to give the desired representations of it. These observations will be found useful in obtaining stereoscopic views of the structures in carpentry and ship-building. CHAPTER XII. APPLICATION OF THE STEREOSCOPE TO NATURAL HISTORY. In treating of those objects of natural history which enter into the composition of landscape scenery, such as trees, plants, and rocks, we have pointed out the method of having them accurately drawn for the stereoscope; but it is to the importance of stereoscopic photography in natural history as a science that we propose to devote the present Chapter. When we reflect upon the vast number of species which have been described by zoologists, the noble forms of animated nature, whether wild or domesticated, and the valuable services which many of them perform as the slaves of man, we can hardly attach too much importance to the advantage of having them accurately delineated and raised into stereoscopic relief. The animal painters of the present day,—the Landseers, the Cowpers, and the Ansdells, have brought this branch of their art to a high degree of perfection, but the subjects of their pencil have been principally dogs, horses, deer, and cattle, and a few other animals, with which they are well acquainted, and specimens of which were within their reach. To give accurate representations of giraffes, hyænas, and the rarer animals which are found alive only in zoological gardens and travelling caravans, is a more difficult task, and one which has been necessarily intrusted to inferior hands. In this branch of his art the photographer is perplexed with the difficulty of arresting his subject in a position of repose and in the attitude which he requires. But this difficulty will diminish as his materials become more sensitive to light; and means may be found for fixing, without constraint, certain animals in the desired position. We have seen the portrait of a dog taken with such minute accuracy that the slightest trace of any motion could not be perceived. Its master directed his attention to a piece of bread, and he stood firmly waiting for his reward. Considering truth as an essential element in all photographs, we are unwilling to counsel the artist to have recourse to a large lens for the purpose of accelerating his process by seizing his restless object in a single instant of time; but what cannot be tolerated in the human form may be permitted in animal portraiture as a necessary evil.