198 



Mr. D. Gill. On the Presence of Oxygen 



the butt end of the telescope, a cast-iron plate attaching to the flange 

 on the other end of the cylinder carries either one or other of the 

 two prism boxes. 



The slit-slide and 60° prism for reflecting the comparison spark on 

 the slit (made on the plan of the Lick spectroscope) are contained in 

 a strong cast-steel box which is permanently attached -to one end of 

 the collimator tube. This latter is a very strong solid drawn steel 

 tube, the external surface of which has been turned and finished to a 

 true cylindrical form, and it rests in proper geometrical bearings 

 formed in strong cast-iron diaphragms, which are fitted inside the 

 cylindrical body of the instrument. 



A powerful slow motion permits the collimator to be slid along its 

 axis so as to focus the slit upon the image of the star at any required 

 reading of the focussing scale. The object-glass of the collimator is 

 mounted on the end of another steel cylinder which also rests on 

 geometrical bearings inside the outer collimator tube, and it is also 

 provided with fine slow motion and a focussing scale. Both these 

 scales are illuminated at will by small incandescent lamps, and are 

 read by microscopes which are accessible from the outside. 



The whole instrument can be enveloped in felt to prevent any but 

 very slow change of temperature. 



The comparison-spark apparatus is arranged with wide angle object- 

 glasses, in such a way that if the image of the spark shines on the slit 

 the object-glass of the collimator must be full of light. Numerous 

 trials in all positions of the instrument have invarably given photo- 

 graphs of the lines of the comparison spectrum of iron rigorously 

 coincident with the corresponding lines of the solar spectrum, the 

 latter being obtained by exposing the slit in diffuse daylight. 



The camera end with its focussing and tilting adjustments can be 

 attached to either telescope by a flange with bayonet joint. The 

 focussing scale is divided to 1/10 mm., and the amount of tilt of the 

 plate-holder is measured on a graduated arc. 



As the large telescope is fitted with an object-glass prism of 24 inches- 

 aperture (which, when the slit spectroscope is in use, is folded back in 

 the manner shown in the frontispiece of Mr. McClean's 1 Spectra of 

 Southern Stars '), heavy counterpoises are required to balance the tube 

 about the declination axis if the spectroscope is not attached. 



In designing the slit spectroscope I was thus not limited by the neces- 

 sity for lightness in its construction. The complete instrument weighs 

 400 lbs., beiug almost exactly the equivalent of the counterpoises and 

 focussing slide and camera, which are removed for its adaptation. In 

 every detail the fittings of the spectroscope are designed with the 

 necessary geometrical limitations of freedom and no more, so that no 

 shake nor variation of adjustment can arise from imperfection of work- 

 manship ; that is to say, all the adjustments depend on adequate spring 



