SOUECES OF ILLUMINATION 103 



position. This arrangement is perhaps the simplest and most 

 efficient that can be obtained for photo-micrographic work, 

 as it is obviously only necessary, once and for all, to set the 

 pin-hole camera in the position that coincides with the 

 optical axis of the microscope. On starting the arc, there- 

 fore, no further adjustment is necessary with the microscope 

 itself, the subsequent observations to be made being only 

 those through the pin-hole camera. It will be found in 

 practice that any adjustment of the arc, whether for 

 position or length, is only necessary at intervals of one to 

 two minutes, so that in any ordinary length of exposure 

 an adjustment made before the exposure is commenced will 

 be all that is necessary to ensure the arc remaining in its 

 proper condition for a sufficient length of time. When the lamp 

 is started no attempt should be made to take a photograph 

 with it for a minute or two, as probably owing to the fact that 

 there is a considerable amount of air occluded in the carbons, 

 the light does not become steady until it has been driven off. 

 The pin-hole camera will always clearly indicate when the light 

 is ready for use. 



It does not come within the province of this work to discuss 

 the construction of automatic arc-lamps in detail, so that only 

 two well-known types will be referred to. 



In the lamp supplied by Messrs. Carl Zeiss, the carbons 

 are automatically fed towards one another, but there is a con- 

 trivance by which the positive carbon is kept in an approxi- 

 mately constant position, and by adjusting the screw which 

 projects at the back of the body the position of the positive 

 crater may be varied ; the automatic arrangement then 

 ensures it being kept in its correct position. There are also 

 two screws at the side of the body of the lamp by which lateral 

 adjustment is made. 



In the automatic lamp supplied by Messrs. Leitz, the method 

 is adopted of placing the carbons at right angles to one another ; 

 the object is to project the light from the positive carbon 

 directly towards the microscope, the negative carbon being 

 sufficiently low down not to intercept any of the required 

 light. 



This arrangement requires that the positive carbon should 

 be as small as possible in diameter for the current used, so that 



