THE CONSTITUTION OF THE ELECTRIC SPARK. 1<J5 



electrodes illuminated by the spark are in perfect focus. By a few successive trials, 

 in which the electrode box and lens A alone were moved, and in such a way that 

 the latter was shifted through half the distance of the former, this could always be 

 secured, and at the same time the lens kept half-way between spark and slit. 



The lens A had now to be temporarily removed, to make sure that the axis of the 

 collinmtor pointed to the centre of the spark gap. For this purpose the collimator 

 lens was covered by a piece of cardboard with a central slit about 2 millims. wide, and 

 the electrode box shifted sideways until the maximum amount of light passed through 

 this slit. Replacing the lens A into its previous position, it is easy to ascertain that 

 the adjustment of distances has not been disturbed. The adjustment being complete, 

 the correct position of collimator was permanently secured by pouring melted paraffin 

 wax round the tripod screws which carry it. The lens A was fixed in the same way, 

 but a lateral motion of the electrode box must be allowed, so as to correct the small 

 displacements which necessarily occur when one set of electrodes is replaced by 

 another. 



The prism is placed in the usual way in its proper position in front of the collimator ; 

 we worked in a position of minimum deviation for a wave-length of about 4 '3 10~*. 



The only remaining adjustments are those of the camera lens and disc. The lens 

 D was fixed to a brass tube which could te made to slide in a collar by means of a screw. 

 The lens was placed so that it was completely illuminated by light passing through the 

 collimator and prism ; this is easily tested by an eye placed at the focue of the lens. 



When the position of the disc had been accurately fixed, so that the spectrum was 

 formed at the proper place, a film, which had to be sacrificed for the purpose, was placed 

 on the disc just as during an experiment. A pointed rod was now brought into contact 

 with the film and against some prominent spectrum lines like the blue triplet of zinc. 

 The rod was fixed in this position, slightly touching the film, so that the disc could be 

 rotated without appreciable friction. The disc was now removed for the final adjust- 

 ment of focus, an eye-piece being put in position, so that the pointed end of the rod 

 was in its focal plane. If the lines of the spectrum were not in focus, the lens D were 

 'moved until that was the case. The wheel being then replaced into contact with the 

 rod, a perfect focus was secured. 



The different parts of the spectrum were not situated at equal distances from the 

 axis. A slight correction which may in consequence be necessary in the reduction of 

 the observations, requires the knowledge of the point of the spectrum which lies at the 

 minimum distance from the axis, i.e., if the spectrum is horizontal, we want to know 

 the wave-length lying vertically above the axis. This may readily be ascertained by 

 suspending a plumb-line close to the disc passing in front of its centre. The shadow 

 of the string will then show on any photograph of the spectrum which is taken. 



2. Interpretation of Photographs. 



NVith the wheel which was first used, the film was placed round the rim (fig. 1 



2 c2 



