ArrAR.'XTUS AND METHODS. 



13 



insulating material, the face-plates at the ends of the coil being protected by ebonite sheets, and 1300 

 turns of wire were wound on each coil, the extra 100 turns on the two coils more than compensating for 

 the magnetic leakage caused by introducing the copper spiral. 

 With a stream of water flowing through the spiral, the core remains 

 perfectly cool and a current of 14 amperes may be used without 

 serious heating of the wire. This improvement has given an 

 increase of field of about 25 per cent over what could previously 

 be used for long runs with the same magnetic gap. 



The current is controlled by means of two Ruhstrat sliding 

 resistances in parallel and is read to o.i ampere by a Weston 

 millivoltmeter with shunt used as an ammeter. 



3. The Spectrograph. 



The spectrograph which was used in this investigation was 

 described briefly in the general account of the Pasadena labora- 

 tory published in 1908 (53) . It is of the Littrow or autocollintating 

 t)-pe, placed vertically in a well 30 feet (9.1 m) deep. The design 

 of tliis spectrograph was worked out during the early solar inves- 

 tigations on Mount Wilson and the first instrument in the obser- 

 vatory equipment was made by William Gaertner of Chicago, and 

 has been in use for over three years as a part of the 60-foot tower 

 telescope on Mount Wilson. A description was published in 

 Contributions from the Mount Wilson Solar Observatory (54) . When 

 the physical laboratory in Pasadena was equipped in 1908, an 

 exact duplicate of the mountain spectrograph was obtained from 

 Gaertner, with the addition of holders for lens and plane grating 

 to give a focal length of 13 feet (4 m) when desired, as well as 

 the full focal length of 30 feet (9.1 m). 



The details of the mounting of the spectrograph can be seen 

 from the drawing in Fig. 3 and from the photograph of the upper 

 end (Plate II). The well is made water-proof with a lining of 

 brick, several layers of tarred building paper, and cement plaster, 

 the dimensions being 30 feet (9.1 m) below the floor of the labora- 

 tory and 8.5 feet (2.6 m) in diameter. Since the well was thor- 

 oughly dried out, no moisture has appeared to come through the 

 walls. The cover of the well is of reinforced concrete, with two 

 openings. A circular opening at the east side is inclosed by a 

 cement ring 70 cm high and no cm outside diameter, which 

 supports the metal top of the spectrograph. Entrance to the 

 well is provided for by an opening at the south side closed by a 

 wooden cover, from which a vertical iron ladder leads to the 

 bottom. Attached to the iron ladder is a stout wood platform, at 

 such height that the parts of the spectrograph for the 13 feet 

 (4 m) focus can be conveniently adjusted. 



The spectrograph consists essentially of a skeleton steel frame 

 50 cm square, at the top of which is a circular cast-iron plate on which is the slit and holder for the 

 photographic plate, while below, the objectives and gratings are supported in the steel frame at the 



