652 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
regulator; (5) the arc-light can be centered both horizontally andfverti- 
cally ; (6) the size of the arc is regulated by one handle only ; (7) the 
whole regulator can be pushed about 14 cm. horizontally ; (8) the entire 
apparatus is outside the camera. 
The regulator is mounted on an aluminium plate, which forms the 
back wall of the camera : it is fastened thereto by the milled heads 
rrrr (fig. 156) after the removal of another similarly fastened back wall 
carrying the lime-light. Each back wall has a circular sinking 7c, near 
which is the screw / for centering the arc vertically ; the clamp g fixes 
the regulator when centered ; the screw e controls the horizontal center- 
Fig. 156. 
ing of the arc. On loosening a, the whole regulator, including c, li, d , is 
moved backwards or forwards by gripping c. This last movement 
therefore serves to bring the image of the arc into the rear principal 
plane of the objective. Rotation of the screw d regulates the size of the 
arc light from time to time. Fig. 157, showing the inner side of the 
back plate, will give a clear idea of the action of the various parts. The 
horizontal and vertical adjustments by rotation of e and / are really 
circular movements about centres n and m. For controlling the size of 
the light the rotation of d is transmitted through the conically toothed 
wheels o, jo, to two sliders q and s, which mutually approach or recede. 
The carbons are inclined at 45° to the vertical, so as to get the best 
