8 Mr. G. A. Hemsalech on the Relative Behaviour 



mounted at the end of a long box-camera, and the colli- 

 mator provided with a platino-iridium slit, made by Adam 

 Hilger. All these parts were then mounted and securely 

 fixed in a strong teak-wood case. The instrument was used 

 in a vertical position, with the slit horizontal. 



Special attention was paid to the projection lenses, for it 

 was essential that the image of the source of light formed on 

 the slit should be as perfect as possible. A Zeiss objective 

 of 25 cm. focal length was used for projecting an image of 

 the flame, and a Ballmeyer portrait objective for the com- 

 parison spark. All the various methods and manipulations 

 used in adjusting and focussing the spectrograph and in the 

 centreing and waxing down of all the optical parts, including 

 the projection objectives, have been fully described in my 

 thesis *. 



Since the low-pressure air-coal gas flame burns vertically 

 upwards, its image on the slit is in a direction at right angles 

 to the latter. Sometimes, however, it became necessary to 

 form the image parallel to the slit. This was done by means 

 of a system of two silver-on-glass mirrors, which were placed 

 immediately behind the Zeiss objective and rotated the beam 

 of light through 90°. These mirrors were silvered by 

 Martin's process and coated with a very thin film of celluloid 

 in order to prevent tarnishing by the fumes from the flame. 



Support for burners. 



It was necessary, especially when observing the explosion 

 region with burner No. 1, to adjust the image on the slit 

 with some degree of accuracy ; for this reason the burners 

 were mounted on a support with vertical motion. This 

 support was provided by the unemployed telescope tube of 

 the above-mentioned spectroscope, the focussing tube serving 

 as holder for the burners, the rack-and-pinion movement 

 constituting an excellent fine adjustment. 



Self-induction spark f for comparison spectrum. 



The electrodes, in the shape of rounded-off cones or 

 cylinders, are fixed in a horizontal position, with a gap of 

 about 5 millim. between them : the sparks thus pass in a 

 direction parallel to the slit. They are produced by a small 

 plate condenser of 0*01 micro-farad capacity contained in a 

 lead-lined box filled with paraffin-oil. In one branch of the 



* Hemsalech, TJieses de Doafwat. pp. 83-41 <fc p. 55 (Par's, 19011. 

 t Hemsalech, Comptes JRendvs de I' Academic des Sciences, vol. cxliv. 

 p. 692 (1907). 



