at the total solar Eclipse in 1882 



353 



and is on that account perhaps more likely to be laid off 

 accurately than another. There is, however, no particular 

 advantage in making this restriction, because in designing 

 a series of mirrors for a reflector one of them is sure to be 

 inclined at an angle of nearly 45 and to have an efficiency 

 which is sensibly the same as if the angle were 45. 



The general problem, to construct the principal section of 

 a reflector consisting of a series of conical mirrors when the 

 direction and length of the common focal line are given and 

 the position relatively to a point on this line, of a point 

 occupying a definite position in the line of section of one 

 of the mirrors, is simple. The diagram, Fig. 6, shows a 

 construction of this kind. 



The line OP is the axis of the reflector, it is also the 

 direction of the incident rays when the instrument is in use. 

 Make it the axis y of rectangular coordinates with the upper 

 extremity of the focal line, A, as origin : ordinates measured 

 in the direction AB are to be reckoned positive, those measured 

 in the reverse direction are to be reckoned negative. 



Abscissae are to be measured on a line at right angles to 

 OP, and they are positive when measured to the right. 



Join BB l ; and through , draw a line O l P l parallel to OP, 

 and on it lay off B^A^ = BA. 



Join AA l and produce it to a point B^ so that A 1 h\ = A/>. 



u. 23 



