38 ILLUMINATION. 



The plumb-line is attached to the top of the needle 

 H, and the brass plate G moved till the lead of the 

 plumb-line hangs directly over NS. At the same 

 time the card E, (which should be very accurately 

 made), is placed with its upper extremity at E, the 

 lower side being parallel with NS, and the needle H 

 made parallel with the line corresponding to the 

 latitude of the place on the card. H now points to 

 the pole. 



During these adjustments, the level of the appar- 

 atus, and the north and south positions of the line 

 NS should not be disturbed. The position of the 

 needle should finally be tested by seeing that the 

 plumb line hangs directly over the N extremity of 

 NS. The silk cord is placed round the wheels on 

 the clock and needle, so that the mirror revolves in 

 the direction of the sun's motion when the clock is 

 set going. The sun's rays are directed on the mirror 

 L by the mirror K, and as L can be moved in all 

 directions, the reflected rays may be made to occupy 

 any position. 



This instrument has been described for the benefit 

 of those who have the time and ability for the manu- 

 facture of their own instruments, and because it 

 cannot be purchased ready made. It was found 

 sufficiently accurate for exposures of moderate 

 length, but like all instruments of the kind it re- 

 quires considerable care and practise to adjust. 

 When once fixed, its position should be left un- 

 changed as a separate adjustment every time it is 

 required will be found very troublesome. 



