PROCEEDINGS OF THE SOCIETY. 
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hand-fed arc lamps in use, but the point to which he had endeavoured 
to direct attention was that hitherto no means of accurately observing 
the crater and measuring the length of the arc had been available, but 
with a pin-hole camera such as he had described this was efficiently 
provided. 
The President inquired what light was focused upon the object in 
using the Microscope, also what was about the actual size of the spot of 
light upon the positive carbon. 
Mr. Barnard said the direct light from the carbon point was used ; 
what came through the pin-hole was merely used to indicate the position 
of the light. The diameter of the brightest part of the crater was about 
1 mm. 
Mr. C. L. Curties said that in the lamp used at Dr. Symes Woodhead’s 
laboratory on the Thames Embankment, they had crossed lines instead 
of a hole, and the crater was kept central to these. 
Mr. T. A. B. Carver had recently seen a large arc lamp at the Royal 
College of Surgeons, but was surprised to find that, although it was 
equipped with a lens casting an image of the arc upon a coloured screen, 
no reference marks were provided by which the true position and con- 
dition of the arc could be observed. 
Mr. C. L. Curties said that it was originally provided with crossed 
wires and screen. 
The President said that he had not used the electric arc for photo- 
micrography, but only the lime light. This was, however, not the 
ordinary lime light, but, as he had often described, was arranged so as 
to produce a small bright steady source of light. He - got this by 
using only about 1-inch pressure of gas and very hard lime, and in this 
way he obtained a comparatively steady light, although some of his 
negatives had been spoilt through some little irregularities in the lime 
or the pressure. One thing he liked the lime light for was with respect 
to the length of the exposure, as it was so much more easy to regulate a 
5- or 10-second exposure than a 1/2-second one; but of course the 
electric arc had its advantages and was extremely clean. For this pur- 
pose he did not use the gas direct from the cylinders, because it was 
simply impossible to regulate it properly, but he worked with gas- 
holders which were filled from the cylinders. 
Mr. Beck asked what optical appliance, if any, was used between the 
light and the substage condenser. 
Mr. Barnard said he had used it both with and without a supple- 
mentary lens ; an ordinary plano-convex lens answered every purpose. 
The slide of typhoid bacilli had been taken with the lamp, using a 
current of 6 amperes, giving a light of from 700 to 800 candle-power. 
A screen of a saturated solution of bichromate of potash was used. 
The image was projected 7 ft. without an ocular, and the exposure was 
6 seconds. The exposure could be varied by changing the pressure, and 
thus varying the size of the source of light. 
The President thought it was highly detrimental to the accuracy of 
a photomicrograph to work without the eye-piece. The greatest distance 
at which he had worked with an eye-piece was 6 ft. 6 in., and he thought 
it was hardly possible to exceed this with advantage. 
The President said he was sure that the Fellows who were present 
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