202 REPORTS ON INVESTIGATIONS AND PROJECTS. 



man effect in the red and yellow with the new magnet, and three weeks to 

 photographic work on the spectrum of the iron arc for Mr. St. John. He 

 has also carried on miscellaneous experimental work under the following 

 heads: Mapping of magnetic fields of a series of spiral conductors lying 

 in a plane, for Professor Stormer ; photographing the spectrum of air with 

 the quartz spectrograph ; determination of the state of polarization of light, 

 originally circularly polarized, after reflection from the two mirrors in the 

 tower telescope; preliminary tests of a machine designed to produce a con- 

 stant angular velocity. 



The instrumental equipment of the laboratory has been increased by the 

 purchase or construction of several pieces of apparatus, the most important 

 being a large electro-magnet of the Weiss type, built by the Maschinenfabrik 

 Oerlikon, of Zurich. The magnet is water-cooled, of 120,000 ampere-turns, 

 and consumes 16 kw. when excited to full capacity. The maximum field 

 obtainable has not been accurately determined, but the magnet furnishes 

 from 30,000 to 40,000 gausses with the rather large pole separation required 

 in the study of the Zeeman effect. The magnet has proved of great value, 

 both for the ordinary high-field photographs of the Zeeman effect and for 

 special experiments in weak fields. For this latter purpose it is superior 

 to a smaller instrument on account of the large space available between its 

 poles in which the field is very uniform, while the facility of its adjustment 

 for pole-distance, angle between line of sight and field, etc., adds greatly to 

 its usefulness. 



A new direct-current generator set of 17 kw. was made up by the pur- 

 chase of a 25-horsepower motor, which was connected with a dynamo already 

 owned by the Observatory. This generator supplies power for the new 

 electromagnet and has proved very useful by reason of its large capacity. 



In addition, a small direct-current generator supplying 500 volts with cur- 

 rents up to 5.5 amperes has been installed for certain work with the electric 

 arc requiring moderately high voltage. 



A 4-inch Rowland concave grating of i meter radius has been mounted 

 in a spectrograph of special design for the photography of the spectrum 

 under low dispersion, including the ultra-violet region. As the instrument 

 is adjusted, a first-order spectrum approximately normal from A 2000 to 

 A 8000 is obtained. The long range of spectrum photographed at a single 

 exposure is often a distinct advantage. As used with the electric furnace, 

 simultaneous photographs are made with this instrument and with the Lit- 

 trow spectrograph, thus obtaining for the same condition of the light-source 

 a long range of wave-lengths, together with a high-dispersion plate for a 

 selected region. 



A concave grating of 15 feet radius has been obtained, for which a mount- 

 ing is being designed. This spectrograph will in general follow Rowland's 

 plan, but will be placed vertically in the laboratory well. The advantages of 

 a concave grating of moderately high dispersion will thus be combined with 

 the features which have given good results with the plane gratings. 



