DIMENSIONS ; but the common idiom refers to that which the mathe matician calls, for distinction, apparent magnitude. It is correct, in the common meaning of the term, to say, that a man at a little distance from the eye is larger than a remote mountain. In thus judging of objects, the angles which they subtend at the eye furnish the means of comparison. Experience, derived from the combination of sight and touch, teaches us how to make those deductions which are necessary before we can learn the absolute from the apparent magnitude.
It is soon found that an object, as it recedes, grows smaller ; that is, subtends a less angle. It is also seen that the recess is accompanied by a lose of brightness and distinctness. The former is a consequence of the loss of light which takes place in its passage through the air ; were it not for this, the same object would be equally bright at all distances ; for though the quantity of light which enters the eye is diminished by increase of distance, yet the surface from which the light appears to proceed is diminished in the same proportion. The oss of distinctness is a consequence, first of the loss of light, next of the d tenet proportion in which different colours are lest : the effect of the interposed atmosphere amounting to laying on more or leas of the blue colour of the atmosphere over the whole. Our perception of magnitude depends both on the subtended angle and on the distinct ness : we learn from experience, that of two objects seen under the sane angle, the less distinct, as being the more distant, must be the larger. That habit is our guide can readily be shown by producing
instance. in which we are deceived, the object being either such as is not commonly seen, or seen under unusual circumstances. A colossal statue mounted on a column does not suggest the idea of a man of unusual size to persons in general, unless when some persou mounts the same height and affords means of comparison. In a fog, which diminishes the distinctness of objects, but does not affect the angles under which they are seen, these objects are sensibly increased in apparent size; and distant hills appear nearer in a clear day than in a hazy one. Those who wear spectacles may satisfy themselves by breathing on the glasses, and watching an object as the moisture evaporates, that increase of distinctness gives apparent approximation.
The angle subtended by an object is inversely as its distance, which is sufficiently near for common purposes, when angles are small, which is generally the case; and a man of 6 feet high, at the distance of 100 feet, is seen under an angle of 3' 26'. The snn is seen under an angle of 32% and the moon under an angle of 244' to 33r.