20 



Photograpliy at Greenwich Observatory. 



A diagram of the apparatus will render this more intel- 

 ligible. In the accompanying drawing (Fig. 1) only the 



principal parts of the arrange- 

 ment are represented. AB 

 is the magnet, C the cylinder, 

 D the timepiece, B the flame, 

 F the mirror, Gr the cylindri- 

 cal lens, and H the photo- 

 graphic trace. Originally camphine 

 lamps were used for the photographic 

 light ; but for many years past naphtha- 

 lized gas has been substituted, and is 

 equally efficient, while much more 

 manageable and cleanly. The gas, 

 previous to burning, passes through a 

 bos divided into compartments con- 

 taining naphtha, which is kept gently 

 warmed by water beneath it, heated by 

 a small flame. 



Each cylinder is covered with a 

 double case of blackened zinc, having 

 a slit in the direction of its axis, vertical 

 or horizontal, as the case may be, in 

 front of which is placed the cylindrical lens. The whole 

 course of each beam of light, from the flame to the magnets, 

 and thence to the cylinders, is also enclosed in tubes of 

 blackened zinc, keeping out all extraneous light. 



As neither the glass cylinders can be expected to be very 

 perfect in shape, nor to rotate very sjnnmetrically, nor the paper 

 to be always exactly alike in size, the following contrivance to 

 obtain a base line from which to measure the departure of the 



