1 4 INFLUENCE OF A MAGNETIC FIELD UPON THE SPARK SPECTRA OF IRON AND TITANIUM. 



proper levels for the focal lengths desired. The weight of the frame is supported by a concrete pier 

 placed at the bottom of the well. This pier carries an iron plate with a spherical cavity, into which 

 fits a lubricated hemisphere on the lower end of the spectrograph frame. The iron plate at the upper 

 end of the instrument fits loosely inside a circular iron casting imbedded in the concrete ring already 

 described. The whole spectrograph turns easily about a vertical axis by means of a gear and pinion in 

 the outer casting. A simple clamping device holds the instrument against accidental turning when in use. 



The sUt of the spectrograph, 51 mm long, is placed on the end of a brass tube sliding within another 

 tube attached to the iron top. The divided head regulating the width of slit is graduated to read 0.025 mm. 

 For strong light sources, a slit width of 0.075 m ^^.s regularly used. When the 30-foot arrangement 

 is in use, the light passes from the sht to an 8-inch (20.3 cm) visually corrected objective by Brashear, 

 which lies horizontally in a holder capable of being moved vertically for focusing by turning a rod pass- 

 ing to the top of the spectrograph and rotated by a hand-wheel. A metal box to hold a plane grating 

 is just below the lens. A rod, geared to the grating box and passing above to a second wheel at the 

 top of the instrument, permits the rotation of the grating about a horizontal axis to obtain the order 

 or region of spectrum desired. Scales which show the position of the lens and the inclination of the grat- 

 ing can be read by a small telescope at the top of the instrument when illuminated by incandescent 

 lamps turned on from above. The fight reflected by the grating passes again through the lens and the 

 spectrum is brought to a focus above, the middle of the photographic plate lying in the same plane as 

 the slit. The holder carrying the plate rests in an iron frame supported at its center so that by tilting 

 the plate-holder good focus can usually be obtained over the whole of the plate, which is 17 inches (43 cm) 

 long and 3.62 inches (9.2 cm) wide. The plate-holder can also be moved parallel to itself by means 

 of a rack and pinion to permit the photographing of successive spectra. Two shutters, sfiding horizon- 

 tally, are placed 7.5 mm below the plate and can be adjusted to shut out all fight except the strip of 

 spectrum, the width of which is regulated by the length of sfit used. Light reflected from the lens sur- 

 faces would reach the plate were it not for these shutters and for the fact that a narrow bar is laid across 

 the center of the lens so as to cut off the reflected rays which would enter through the opening of the 

 shutters. With the 30-foot focal length, a slight inclination of the objective removed the reflections 

 without appreciably affecting the definition. 



The arrangement of lens and grating to give the spectrograph a focal length of 13 feet (4 m) follows 

 the plan of that for the longer focus. The movements of lens and grating are regulated by the same 

 rods wliich control those below. The grating-holder may be moved over to the side of the steel frame 

 and the lens-holder swung back out of the way when the 30-foot arrangement is desired. 



The two plane gratings used during the investigation were a Rowland grating 12.5 cm long and g cm 

 wide, having 568 fines to the milfimeter and a Michelson grating 19 cm long by 7.2 cm wide, having 500 

 fines per mm. The former was used with the 13-foot arrangement for the majority of the plates. The 

 Michelson grating was obtained near the end of the investigation and a number of the later plates were 

 taken with this, which was adjusted for the 30-foot focus. While longer exposure must be used with the 

 longer focus, the large scale is very desirable and the field is much flatter, so that as a rule the whole 

 length of spectrum over a 17-inch (43 cm) plate can be obtained in fair focus, even in the first order. 

 For very weak light-sources, however, the 13-foot arrangement often gives better results, as there may 

 be unavoidable changes in either the source or the spectrograph if the exposure is greatly prolonged. 



The scales of the photographs for the two focal lengths and the several orders used in this work are 

 approximately as shown in the smafi table on the following page, there being a variation in the second 

 decimal place according to the part of the spectrum observed. 



Other important features of the spectrograph are the occulting plate of the sfit, the mirror support, 

 and the polarizing apparatus. Plate II shows the form of the occulting plate. It is of brass, dull silver- 

 plated, and supported on four pins screwed into the top of the spectrograph, so that it is entirely free 



