OPTICAL INSTRUMENTS. 



CHAPTER II. LensesBurning Lenses 

 Polyzonal Lenses. 



Lenses. (4.) A lens is any" transparent 

 substance, as glass, crystal, water, or 

 diamond, having one or both of its sur- 

 faces curved to collect or disperse the 

 light transmitted by it. The lenses in 

 general use are made of glass, and are 

 usually called magnifying glasses. Glass, 

 however, does not possess a greater 

 share of the magnifying property than 

 other transparent substances. In fig. 4, 



there are six differently shaped lenses, 

 shown in section. A is called a plano- 

 convex, from having one side flat, and 

 the other spherically rounded. B is a 

 double convex, and has both sides sphe- 

 rically rounded. When these sides are 

 unequally curved, as at C, it is termed 

 a crossed lens. D is a plano-concave, 

 having one side spherically hollow. E 

 is a double concave with both sides 

 hollow. F is a Meniscus (so called 

 from its moon shape), and has one side 

 convex and the other concave. 



Fig. 4. 



(5.) The passage of light, when trans- 

 mitted by a plano-convex lens, is shown 

 mfigs. 5 and 6. Let A (fig. 5.) represent 



Fig. 5. 



a section of the lens, and B an object at 

 an infinite distance, as a star (in the 

 figure a cross is placed to assist the 

 conception). Now, the lines from B to 

 A will represent the rays proceeding 

 from every part of the object B, to every 

 portion of the surface of the lens A, and 

 from the distance of the object they 

 will be parallel to each other. Only five 

 rays are drawn in the figure, to prevent 

 confusion ; but it should be constantly 

 remembered that the light strikes every 

 part of the lens. The first surface of 

 the lens next the object B is flat ; and 

 as all the rays fall perpendicularly on 

 it, and the attraction on each side of 

 the rays is equal, they will pass on in 

 their right-lined direction, till they meet 

 the curved surface of the lens, when 

 all the rays will be bent or refracted, so 

 as to meet in a point F, called the focus 

 of the lens ; but the central ray being 

 perpendicular to the curved surface is 



not bent. This constant law of refrac- 

 tion, or bending, is always observed by 

 rays of light in their passage from one 

 medium to another of diiferent density, 

 whose surface is oblique to their direc- 

 tion ; and the rays of light in passing 

 from a dense medium, as glass, &c. 

 into "a less refractive medium, as air, 

 will be bent so as to form a greater 

 angle with a perpendiular to that sur- 

 face, than it had at first. Let the dotted 

 line c d be a perpendicular to the sur- 

 face of the lens at the point e. Now, 

 the angle a e c, called the angle of inci- 

 dence, is less than the angle F e d, called 

 the angle of refraction, as is shown by 

 continuing the ray a to i. 



(6.) But whenever light strikes an ob- 

 lique surface in its passage from a rare 

 medium, as air, to a denser, as glass, it 

 will be refracted nearer the perpendi- 

 cular, as shown by fig. 6, in which 

 the lens is reversed. The ray of light 

 a eis there bent nearer the perpendicu- 

 lar c d, as shown by the dotted line 

 e r, and when this ray is transmitted by 

 the plane side, it will be refracted to F, 



Fig. 6. 



