Place "a" of the upper cipher disk under B of the lower disk and notice whether the cipher letters jbfo--the first group--are intelligible. They give "sawn," continue this for "saw," the first three letters, may be the text word. Now the next group is B N Y R and these give A O D K. We know that A does not represent B because the first 8 cipher letters give the meaningless letters "sawnaodk." Turn "a" to C and we have for the first group T B X O, which is without meaning. Turning "a" to D we get U C Y P, a meaningless jumble. Turn "a" to E and we get V D Z Q, which is meaningless. Now turn "a" under F and we find that JBFO mean "Wear," which, so far at least, gives us a part of a word, or the word "We" and part of another word. We continue to the next group B N Y R, which gives us "esho." We now have these letters "Wearesho," which at a glance we read "We are sho;" continuing to the next group O M R A the cipher disk gives us "rtof," and we read "We are short of" and know we have found the key letter, and the information hidden in the cipher is ours. Continue deciphering with "a" under F until the end of the message. Sometimes the key letter is changed after two, three, or four letters. It is a matter of minutes only to run through the alphabet and learn the meaning of a message so enciphered. CHAPTER VII. FIELD GLASSES AND TELESCOPES. Reflection--refraction--lenses. [Illustration: FIG. 20.] When light falls on a transparent body, part is reflected and part is refracted. The angle which the ray makes with the normal, or perpendicular, to the surface at the point of contact is known as the angle of incidence, and the angles which the reflected and refracted rays make with the same normal are known respectively as the angle of reflection and refraction. The reflected ray makes the same angle with the normal as the incident ray, while the refracted ray, when passing from a rarer to a denser medium, is bent toward the normal, and vice versa; the denser the medium into which the ray passes the greater is the deviation. This law allows us at once to understand the action of a lens, which may be defined as a transparent medium that from the curvature of its surface causes the rays of light traversing it to either converge or diverge. The ordinary lenses have either spherical surfaces or a combination of spherical and plane surfaces. This combination will give rise to six classes (fig. 20): (_a_) Double convex; (_b_) plano convex; (_c_) double concave; (_d_) plano concave; (_e_) converging, and (_f_) diverging meniscus. Those lenses which are thicker at the center than at the edges are converging or concentrating lenses, and those which are thicker at the edges than the center are diverging. FOCUS--OPTICAL CENTER.