LENSES. 



28. Examples of each of these forms of lenses are more or less 

 familiar to every one. Thus the glasses of spectacles used by 



Fig. 18. 



TA 



weak-sighted or aged persons, are usually double convex lenses. 

 Those used by short-sighted persons are generally double concave 

 lenses. 



Spectacles called periscopic are sometimes used. The glasses 

 of these, which 

 are suited to weak 

 sight, are me- 

 niscus, and those 

 adapted to short 

 sight are concavo- 

 convex lenses. 



The eye-glasses 

 of opera-glasses 

 are usually double concave lenses. The object glasses are 

 generally plano-convex lenses, the plane side being turned 

 inwards. 



29. If an object such as o"', fig. 21, be placed before a convex 

 lens, and at right angles to its axis, an image, i"', of it will be 

 produced behind 

 the lens, also at 

 right angles to the 

 axis, inverted in 

 position in relation 

 to the object, that 

 is, the top of the 

 image corresponding 

 with the bottom of 

 the object, and the right side with the left, and vice versa. 



If the object be placed near the lens, the image will be formed 

 at a great distance from it, and will be greater than the object in 

 its linear dimensions in the same proportion as its distance is 

 greater than that of the object from the lens. 



This will be evident by inspecting the figure. The length of 

 the image, i'", is evidently greater than that of the object, o'", in 



95 



