executed in varieties of Jena glass which bring perturbing hues to the vanishing point. In their achromatic, or color-free, lenses and their cameras, or dark chambers, our photographic instruments much resemble the eye. Indeed, it may be that when we see an object the impression is due to a succession of fleeting photographs, following each other so rapidly on the retina as to seem a permanent picture. The eye, furthermore, is stereoscopic; by uniting two images seen from slightly differing points of view, it enables us to judge of size, solidity, and distance. [Illustration: A is flint glass, B is crown glass. They unite to form an achromatic lens.] [Illustration: B, C, F, prism crown glass. C, D, F, prism flint glass, more dispersive than crown glass. The beam S emerges as E, but little decomposed. Were A, B, F a prism of one kind of glass, E would be much decomposed.] Limbs and Lungs as Prototypes. Long before there was a philosopher to classify levers into distinct kinds, the foot of man was affording examples of levers of the first and second orders, and his fore-arm of a lever of the third order. Ages before the crudest bagpipe was put together, the lungs by which they were to be blown, and the larynx joined to those lungs, were displaying a wind instrument of perfect model. The wrists, ankles, and vertebrae of Hooke might well have served him in designing his universal joint. Indeed weapons, tools, instruments, machines, and engines are, after all, but extensions and modified copies of the bodily organs of the inventor himself. [Illustration: _Lever of the 1^{st} order._ _Lever of the 2^{nd} order._ _Lever of the 3^{rd} order._ P, power. F, fulcrum. W, weight.] Canals have called forth the ingenuity of an army of engineers; ever since the first heart-throb, the circulation of the human blood was exemplifying a system in which the canal liquid and the canal boats move together, making a complete circuit twice in a minute, distributing supplies wherever required, and taking up without stopping return loads wherever they are found ready. The heart, with its arteries and veins, forms a distributing apparatus which carries heat from places at which it is generated, or in excess, to places where it is deficient, tending to establish a uniform, healthful temperature. To copy all this, with the ventilating appliances prefigured in the lungs, is a task which in our huge modern buildings demands the utmost skill of the architect and engineer. [Illustration: Arm holding ball.] Postal and Telephonic Service.