Application of the Electric Arc to Photomicrography . 173 
apparatus we designed as part and parcel of the feeding mechanism, 
providing it with a universal motion and a clamp, so that, the light 
having been adjusted to be truly central, the indicator can be placed 
in a position to then read correctly upon the ground glass. In fig. 27 
the lamp is shown, with pin-hole camera attached, at right angles to 
the optical axis. 
The question of inclination of the carbons to the vertical was 
investigated by means of an apparatus giving revolution of the arc 
about a horizontal axis passing through itself. 
The critical inclination is obviously the greatest at which none 
of the light from the crater is cut off by the negative carbon, when 
the length of arc has been adjusted to the greatest that gives con- 
stancy in the position of the crater. 
The critical angle we found to be about 27° to the vertical, and 
at this angle the two essential conditions are secured, viz. the greatest 
amount of light was thrown forward from the crater of the positive 
carbon and the light from the negative is thrown to the back, and 
consequently eliminated as a second and harmful source. 
It is of course understood that the current employed throughout 
these experiments has been a continuous one. It remains to state 
how ill the alternating current adapts itself to such an application. 
None of the conditions which our experiments with continuous 
current showed to be so primarily essential are present. In the first 
instance, the arc can never be adjusted to give that constancy of one 
incandescent source which we were led to place at the root of any 
success in the use of the arc. Again, the carbons are equally incan- 
descent, so that it is impossible to regard the arc as a single source 
of illumination. These essential difficulties are further regardless of 
minor physical inconveniences, such as the noise, which, although 
they might not form barriers to its use, are nevertheless absent with 
continuous current. 
We venture to think that, could the electric arc be regarded as a 
reliable source of light, it would be much more largely employed in 
optical apparatus where constancy of position and intensity of the 
source of light are essential, and can only express the hope that our 
results demonstrate the ease with which these conditions can be 
accurately secured. 
