170 
Transactions of the Society. 
III. — The Application of the Electric Arc to 
Photomicrography. 
By J. Edwin Barnard, F.R.M.S., and Thos. A. B. Carver, B.Sc. 
{Bead 11th November, 1897.) 
In its application to photomicrography the electric arc presents many 
important advantaged. In addition to its inherent simplicity, the 
high degree of intensity and the smallness of the area of the incan- 
descent source, effect corresponding improvement of the image, and 
render short exposures practicable. Photographic results which 
would not he available with other sources of illumination can further 
be obtained from nearly opaque objects. It has, however, hitherto 
been employed with small success ; and it was with the object of 
localising the difficulty, and if possible rectifying it, that the investi- 
gation to be described was undertaken. 
The result of numerous trials of the best form of automatic lamp 
available showed that, although the arc was working under constant 
conditions so far as the most careful adjustment could secure, the 
photographs obtained were subject to wide variation. Not only did 
the results of equal exposure demonstrate fluctuations in the intensity 
of the light, but shadows and diffraction phenomena were produced 
which were only to be attributed to decentration of the illuminant. 
The difficulty was even accentuated by the fact that at times the most 
perfect results were obtained ; but their reproduction was a matter 
entirely of chance. Our first experiments were therefore devoted to 
the analysis of the behaviour of the arc when thus controlled. The 
method we adopted was to project an image of convenient size upon 
a screen provided with lines, in reference to which the movement of 
the image could be referred. 
Our observations showed that, although working under apparently 
constant conditions, the arc was subject to wide fluctuations, on account 
of the movement of the carbon points due to the feeding mechanism, 
and of the incandescent point travelling round the carbon as the 
result of the variations in the length of the arc. In order to test the 
true effect of these variations, the automatic mechanism was replaced 
by a simple form of hand-fed apparatus, in which absolute positive- 
ness and steadiness of movement was secured, so that the conditions 
observed could be reproduced and studied at length. 
No further experiment was, however, necessary to show that in 
the unsteadiness of the feeding motion of the carbons lay part at least 
of the decentering action to which were to be ascribed the variations 
we had observed in the photographic result. 
