254 FERGUSON'S LECTURES. 



LECT. lines will be perpendicular to the surface of the mirror. 



^X^^, Make the angle C Ad equal to the angle D A C, and 

 draw the right line A d for the course of the reflected 

 ray DA : make the angle C c d equal to the angle DC C, 

 and draw the right line c d for the course of the reflected 

 ray Dd: make also the angle C Bd equal to the angle 

 D B C, and draw the right line B d, for the course of 

 the reflected ray D B. All these reflected rays will 

 meet in the point d, where they will form the extremity 

 d of the inverted image e d, similar to the extremity D 

 of the object D E. 



If the pencils of rays Ef, E g, Eh, be also continued 

 to the mirror, and their angles of reflection from it be 

 made equal to their angles of incidence upon it, as in 

 the former pencil from D, they will all meet at the point 

 e by reflection, and form the extremity e of the image 

 e d, similar to the extremity E of the object D E. 



And as each intermediate point of the object, between 

 D and E, sends out a pencil of rays in like manner to 

 every part of the mirror, the rays of each pencil will 

 be reflected back from it, and meet in all the interme- 

 diate points between the extremities e and d of the 

 image ; and so the whole image will be formed, not at 

 i, half the distance of the mirror from its center of con- 

 cavity Cy but at a greater distance, between i, and the 

 object DE; and the image will be inverted with re- 

 spect to the object. 



This being well understood, the reader will easily see 

 how the image is formed by the large concave mirror of 

 the reflecting telescope, when he comes to the descrip- 

 tion of that instrument. 



When the object is more remote from the mirror than 

 its center of concavity C, the image will be less than the 

 object, and between the object and mirror : when the ob- 

 iect is nearer than the center of concavity, the image will 

 be more remote and bigger than the object : thus, if D E 

 be the object, e d will be the image ; for, as the object 



