celestial bodies, being refracted by the atmosphere r 13. What occa- 

 sions twilight ' 14. How would the heavens appear if there were no 

 atmosphere ? 15. Illustrate the reflection of light by fig. 29. Engr. III. 

 16. Refraction of light by fig. 29. [Note. Fig. 31. is' a vessel with a 

 flower in water at the bottom, seen by the eye in the direction of the 

 rays which enter it. This experiment, and many others, may be easily 

 performed.] 



LESSON 32. 



Different hinds of Lenses. 



Diverge', rays of light coming from n point, and contmually sepa- 

 rating as they proceed, are said to diverge ; the point is called 

 the radiant point. 



Converge', rays which tend to a common point are said to 

 coiiverge. 



A Beam of light is a body of parallel rays; xi Pencil of rays is a 

 body of diverging or converging rays. 



Cam'era obscu'ra, a chamber darkened ; an optical machine used 

 in a darkened chamber, 



A LENS is a glass ground into such a form as to collect or 

 disperse the rays of light which pass through it. They are 

 of different shapes, from which they take their names. If 

 rays proceed from a radiant point distant as far as the sun, 

 their divergency is so trifling that they may be considered as 

 parallel. When parallel rays fall on a piece of glass having 

 a double convex surface, that ray only, which falls in the di- 

 rection of the axis of the lens, is perpendicular to the surface , 

 the other rays falling obliquely, are refracted towards the 

 axis, and they will meet beyond the lens at a point called its 

 focus. The distance of the focus from the centre of the lens 

 depends both upon the form of the lens, and upon the re- 

 fractive power of the substance of which it is made ; in a 

 glass lens, both sides of which are equally convex, the focus 

 is situated nearly at the centre of the sphere of which the 

 surface of the lens forms a portion ; it is at the distance, 

 therefore, of half the diameter of the sphere. The property 

 of a lens which has a double concave surface is to disperse 

 the rays of light. Instead of converging towards the ray, 

 which falls on the axis of the lens, they will be attracted 

 towards its thick edges, both on entering and quitting it, 

 and will, therefore, be made to diverge. Lenses which have 



