HINTS ON THE STEREOSCOPE, 311 



the line. A single individual, thus equipped, can " tap " a telegraph- 

 line, in the daytime, by receiving the message in the ordinary way ; 

 and at night (when, of course, it would be easier to approach the line) 

 by listening to the- clicking of the armature against the electro-magnet 

 of the instrument. But all these dangers are only of a partial or tem- 

 porary character. By carefully patrolling and testing the line, it can- 

 not be interrupted for any length of time without the damage being 

 observed and repaired. By adopting a secret arrangement that there 

 shall be a certain number of letters in the two or three words at the 

 beginning or end of every message, a dispatch sent by an enemy can, 

 in most cases, be detected ; and again, by employing a cipher alpha- 

 bet, it will be difficult for any one who taps the line to obtain informa- 

 tion from the messages which fall into his hands. 



From this brief sketch of the structure and uses of the field tele- 

 graph, the reader will understand what an important part it plays in 

 modern warfare. On the march it directs the movements of advan- 

 cing columns, on the battle-field it flashes orders and information wath 

 the speed of thought to right, centre, and left, of the immense lines ex- 

 tended over mile after mile of country ; in beleaguered cities it places 

 the whole defense from moment to moment under the eyes of those 

 intrusted with its direction, and it is of no less value in the attack. 

 It is not too much to say that, without this w^ondrous power, it would 

 be almost impossible to direct the movements of the thousands on 

 thousands of men, and guns, and horses, which form the vast armies 

 of Continental Europe. It has eflTected a revolution in military sci- 

 ence, none the less important because it is hidden from the general 

 view, and seldom attracts the attention of even the ubiquitous special 

 correspondent. Armed with all the weapons which inventive genius 

 and mechanical skill can devise, the modern commander has the light- 

 ning also to do his work, and the electric current gliding on its secret 

 path through the wide net-work of cable and wire tells him what is 

 passing each hour in the remotest parts of the theatre of war, and 

 transmits the mandates which decide the fate of nations. — Popular 

 Science Heview. 



HraTS ox THE STEKEOSCOPE. 



By franklin C. HILL. 



THE following diagrams will help to explain the principle on which 

 this instrument acts. The stereoscope-glasses are halves of the 

 same lens, placed with tlieir outer edges toward each other {LL)^ Fig. 1. 

 Rays of light {R R) from the objects (0 0), striking the oblique sur 

 faces of the lens, are refracted outward, toward the focus, and thus 

 reach the eyes {E E) in an oblique direction, appearing to come from 



