If substances such as glass had not been endowed with this property, it would be difficult to understand how the great discoveries in the science of astronomy could have been made, or what information we could have gained respecting those interesting truths so constantly revealed by the aid of the microscope. Numerous instances might be quoted of the value of this latter instrument in the detection of adulteration, and the examination of organic structures. When so many talented and industrious scientific men are at work with this [Page 299] instrument, it is perhaps invidious to point to one singly, though we must make an exception in favour of Professor Ehrenberg, of Berlin, whose microscope did such good service in procuring undeniable proof of the Simonides' fraud; he has made use of it again to detect the thief that stole a barrel of specie, which had been purloined on one of the railways. One of a number of barrels, that should have contained coin, was found on arrival at its destination to have been emptied of its precious contents, and re-filled with sand. On Professor Ehrenberg being consulted, he sent for samples of sand from all the stations along the different lines of railway that the specie had passed, and by means of his microscope identified the station from which the sand must have been taken. The station once discovered, it was not difficult to hit upon the culprit in the small number of _employés_ on duty there. [Illustration: Fig. 286.] The simplest case of refraction occurs in tracing the course of a ray of light through the air, and into the medium water; in this case it passes from a rare to a dense medium, and the fact itself is well illustrated by the next diagram, in which the shaded portion represents water, and the paper that it is drawn upon the air. The line A B is a perpendicular ray of light, which passes straight from the air into and through the water, without being changed in its direction. The line C D is another ray, inclined from the perpendicular, and entering the water at an angle, does not pass in the straight line indicated by the dotted line, but is refracted or bent towards the perpendicular at D E. This fact reduced to the brevity of scientific laws is thus expressed:--When a ray of light falls perpendicularly on a refracting surface, _it does not experience any refraction or change of direction_. When light passes out of a rare into a dense medium, as from air into water, _the angle of incidence is greater than the angle of refraction_. And when light passes from a dense into a rare medium, as out of water into air, _the reverse takes place_, and _the angle of incidence is smaller than the angle of refraction_.