THE CONSTITUTION OF THK Kl.F.CTRIC SPARK. 198 



polished by means of emery paper, and finally by wash-leather. Good polish waa 

 found necessary to prevent sharp projections, which, though they may be small, 

 cause premature discharges before the jars are sufficiently charged. 



A section of the electrode box is shown in fig. 7. It is made of wood and is fixed 

 to a board, RS, capable of being levelled so as to make the spark path as nearly as 

 possible parallel with the slit. The electrodes are fixed to two wooden plates, AB 

 and A'B', which are attached to flint-glass tubing, CD and CD', the glass bein^ 

 carefully varnished. The electrodes, E, E', are connected to the Leyden jars by 

 means of gutta percha-covered wire (No. 18) which passed, as shown in the figure, 

 through apertures in the plates, AB, A'B', and finally through glass tubes, F and F'. 

 The leads from the jars were hooked to loops formed by the wires attached to the 

 poles, the sharp ends of all wires being protected by means of small spheres of 

 sealing-wax. We used in the experiments a standard distance of 1 centim. between 

 the electrodes. The electrodes having been fixed to the plates, their distance was 

 adjusted and the plates secured to the glass rods by means of sealing-wax. 



A few words should be said about the optical arrangements. It was for our 

 purpose most important to have a good image of the spark in coincidence with the 

 slit. As we confined our investigation principally to that part of the spectrum 

 which, with glass prisms and lenses, is photographically most intense, the lenses were 

 specially made for us by Messrs. ZEISS at Jena, the chromatic and spherical aberrations 

 being corrected for the region F to H of the solar spectrum. When it is required to 

 form a spark image on the slit, sufficient attention is not always given to the fact 

 that lenses are generally constructed to be used with a parallel beam of light. The 

 aberrations introduced when objects at small distances are to be focussed, are some- 

 times considerable, and had to be avoided in our experiments. We asked Messrs. 

 ZEISS therefore to make a lens of 4 centims. aperture and 25 centims. focus, which 

 when placed at a distance equal to twice its focal length from the spark, should give 

 an image equal in size to the object within the required limits of the spectrum, 

 free from chromatic and spherical aberrations. The lens sent to us answered all 

 our requirements perfectly. Both collimator and camera lenses had apertures of 

 4 centims., and focal lengths of 46 '3 and 39 '9 centims. respectively. The prism used 

 was made of old flint glass by STEINHEIL, having a refracting angle of 59 28' and a 

 refractive index of l'G227 for sodium light. The extent of the spectrum between 

 F and H on the revolving disc was about 1 '5 centim. 



The general arrangement of the apparatus is shown in a diagrammatic form in fig. 8. 

 E represents the electrode box, A the lens forming the image of the spark on the 

 slit of the collimator B, through which the light passes before it is dispersed by the 

 prism C and focussed by the camera lens D. The parts A, B, C, D were mounted on 

 a slate slab secured to the wall by means of strong brackets. The lathe support 

 which carried the motor M and spinning disc F, was placed so that the image of the 



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