ArrARATUS AND METHODS. 



IS 



from the slit, and about 2 mm above the latter. By moving the V-shaped opening a, by means of the 

 rack and pinion, any length of slit up to 11 mm may be obtained. A further movement brings the 

 double opening b, whose size may be adjusted by the sHding plate c, over the slit. By a proper setting 

 of the scale d, a double comparison spectrum can thus be placed outside that made with the opening a 

 without risk of instrumental displacement, since the plate-holder and all essential parts of the spectro- 

 graph are left untouched. 



Plate II shows the arrangement of the mirror by which the hght coming horizontally from any piece 

 of apparatus in the laboratory is reflected to the slit of the spectrograph. The holder for the mirror, which 

 is of plate glass 12.5 cm in diameter, silvered on its front surface, can be turned about a horizontal axis, 

 and is supported at the lower end of a brass cyhnder. This cylinder 

 can either rotate or move up and down inside a stationary cylinder held 

 in position by three curved iron supports which are screwed to the top 

 of the spectrograph. The mirror may thus be placed in any position 

 necessary to direct the beam into the instrument. As the mirror can be 

 turned in any direction independently of the spectrograph, we may have 

 any desired orientation of the slit with respect to the Hght source, which 

 is usually out of the cjuestion with a spectrograph mounted horizontally. 

 This is a very great advantage in an instrument free from astigmatism. 



For the Zeeman photographs the slit was regularly used parallel to the lines of force of the magnet. 

 In photographing arc and spark spectra in general, it is desirable to use the slit sometimes parallel, 

 sometimes perpendicular to the direction of discharge in the image projected upon it. 



The Nicol prism, by which the Hght polarized in one plane is transmitted to the sHt, is held on a metal 

 platform 3.5 cm above the sHt. The Nicol prism which has been used thus far was loaned by Director 

 Stratton of the National Bureau of Standards. The diagonals of the face are 25 and 30 mm and the prism 

 is 6.5 cm long. It is held in a brass cyhnder having a graduated circle by which the Nicol can be set at any 

 desired angle to the plane of polarization of the incident Hght. A second platform can be placed above 

 the Nicol to hold a Fresnel rhomb when this is desired for the study of circular polarization. 



Since the beam passing through the Nicol is displaced parallel to itself, when the prism is rotated 90 

 to transmit the other Zeeman component the image does not remain on the sUt. The image is then brought 

 back by moving the focusing lens, a simple glass lens of 58.4 cm focal length and 10 cm diameter. After 

 such a change, it was always noted whether the grating was well centered in the beam of Hght, which 

 usually had at least three times the diameter of the spectrograph objective. Although small movements 

 of a focusing lens of the focal length used produce very slight changes in the direction of the beam to the 

 grating, still care was taken never to move the lens when an instrumental displacement of the spectrum 

 Hues could have any disturbing effect. After an exposure with the magnetic field, the only change before 

 starting the exposure for the spark without field was to move the occulting plate above the sHt, so that 

 the comparison spectrum would be on each side of the spectrum taken with the field. The Hght source 

 thus remained unchanged in position, and all parts of the optical system as well as the photographic 

 plate were left untouched. 



The spectrograph remains in adjustment for longer periods probably than with any mounting other 

 than the vertical arrangement in a well. The temperature change at the bottom of the well is entirely 

 negHgible during short periods of time. A recent test showed that during three months in which tem- 

 perature variations of over 15 C were experienced in the laboratory, a thermometer placed beside the 

 grating rose very gradually from i86 to 19 C. During this time the Hghts were frequently turned 

 on to read the adjustment scales, and there were occasional visits by observers to the bottom of the well. 

 Mechanical vibrations are more disturbing. It has been necessary to close the driveway beside the 

 laboratory during e.xposures with the spectrograph, and to take care that no machinery be used which 

 would transmit a vibration to the spectrograph mounting. 



