SPECTROSCOPIC OBSERVATIONS. 67 



attached by a hinge in front of the slit formed a very simple 

 and convenient exposing shutter. 



The slitless spectrograph was mounted on teak boards, which 

 were bolted to the wooden tube of a 6-in. telescope, the 

 collimator being placed at right angles to the axis of the 

 telescope. A total reflection prism placed about 8 in. within 

 the focus of the object-glass reflected the image of the sun into 

 the collimator tube. The dispersion was obtained with a 60 

 dense flint prism* and two compound prisms of about 1J in. 

 aperture. 



The whole apparatus was mounted as low as possible on a 

 rough equatorial stand having R.A. and Dec. slow motions, but 

 no driving gear. The spectrograph end swung freely in a large 

 pit excavated in the north-eastern portion of the observing hut, 

 while the telescope was pointed directly at the sun. 



When the telescope was directed to the place of the sun at 

 mid-totality the object-glass and exposing cap came into a con- 

 venient position near to the other instruments, and about 5 ft. 

 above the floor level. 



The equatorial tele-spectroscope for visual observations was 

 mounted on a pillar made by a packing-case sunk in the 

 ground, and filled with earth and cement. It was placed to the 

 south-west of the other instruments, and in such a position that 

 when directed to the eclipsed sun the eyepiece of the small 

 telescope was in a convenient position for the observer, who y 

 seated near the prismatic camera, could easily manipulate the 

 exposing caps and observe the spectrum at the same time. 



As I had no one to assist me on the day of the eclipse, the 

 three photographic instruments had to be arranged with their 

 exposing shutters near together, so that I could work them all 

 while seated near the tele-spectroscope. 



The plan I proposed for starting the exposures at the right 

 moment consisted in watching in the spectroscope for the 

 actual appearance of the bright lines of the flash spectrum, 

 then exposing simultaneously the prismatic camera and the 

 large spectrograph. After that the slit spectrograph could be 

 opened ; then, after one or two short exposures, a long one 

 could be started with the prismatic camera. During this 

 exposure, which was to last 40 seconds, the large spectrograph 

 was to be moved in R.A. to get into position for the flash 

 spectrum at the end of totality, the plate-holder being at 

 the same time reversed. After closing the 40 -second exposure 

 I was to expose one or two more plates in the prismatic camera 

 for shorter intervals, and then close the slit spectroscope and 

 prepare for the second flash spectrum; exposing the spectrograph 

 and prismatic camera simultaneously as before, and closing the 

 instant the photosphere appeared. 



* Lent to me for this purpose by Mr. Otto Hilger. 



