LENS 



3380 



LENTIL 



When a ray of light passes through a lens 

 it is bent towards the thickest part. Accord- 

 ingly, light rays passed through a convex lens 



L 



i x )t 



.'-' d ^-_ 



LENSES 



The six forms of lenses. The first three are 

 converging lenses, thicker in the middle than at 

 the edges ; the lower three are diverging, thinner 

 at the middle than at the edges : 

 (a) Double-convex both surfaces convex. 

 ( &) Plano-convex one surface convex, one plane. 



(c) Concavo-convex one surface convex, one 



concave. 



(d) Double-concave both surfaces concave. 



(e) Plano-concave one surface concave, one 



plane. 



(/) Convexo-concave one surface concave, one 

 convex. 



tend to meet in a point, or locus. Parallel rays 

 such as a, b, c, d (Fig. 2), passing through the 

 double-convex lens X Y, converge at M, the 

 principal focus. A familiar illustration is fur- 



Fig. 2 



nished by the burning-glass (which see), which 

 brings the sun's rays to a point, so concentrating 

 their heat as to ignite inflammable substances. 

 Images produced by convex lenses are of 

 two sorts real and virtual. A ray passed 

 through the optical center (Fig. 3) of a lens is 

 not refracted. Such rays are called axial. The 

 principal axis L N passes through both the 



optical center and the principal focus H. All 

 rays parallel to the principal axis are so bent 

 as to pass through H. Where R P crosses the 

 axial ray at R, the image at r is found. Simi- 



larly Q is found at q. The resulting image is 

 inverted and real. It is smaller or larger than 

 the object, according as the object is at a 

 distance greater or less than twice the focal 

 length C H. 



When the object is within the focal distance 

 G L (Fig. 4), the image appears on the same 

 side as the object, erect and magnified. Such 

 an image is called virtual. 



The parallel rays from N and R are bent, 

 and meet at the focal point G. The rays L N 

 and L R, being axial, are not refracted. The 

 image N R appears where the axial rays meet 



fig. 4 



the refracted parallel rays, which is on the 

 same side as the object. The illustration shows 

 the use of the double-convex lens as a magni- 

 fying glass. Images formed by concave lenses 

 are erect, virtual and smaller than the object. 

 Thus the reducing glass is exactly the opposite 

 of the magnifying glass, being double-concave 

 instead of double-convex. 



Related Subjects. The following articles in 

 these volumes will be of interest in connection 

 with a study of light : 



Aberration Polarization of Light 



Camera Physics 



Diffraction Reflection 



Ether Refraction 



Fluorescence Spectroscope 



Light Spectrum Analysis 



Microscope Telescope 



Mirror 



LENT, from the old English word lenten, 

 meaning spring, is the season of fasting which 

 begins with Ash Wednesday, forty days before 

 Easter, and ends with Easter Sunday. It is 

 observed by the Roman Catholic, the Eastern 

 and the Anglican churches. In modern times 

 much latitude is allowed in the observance of 

 Lent, the age, health and occupation of indi- 

 viduals being taken into account. See EASTER, 

 for explanation of the above movable dates. 



LEN'TIL, an ancient food plant, one of the 

 first cultivated by man. It is said that the 

 lentil found in Egypt was the pottage for 

 which Esau gave his birthright. It belongs to 

 the family of plants called legumes, which bear 

 seeds in pods, like the pea and bean, and it has 



