ON THE ELECTRICAL PROPERTIES OP PURE SULPHUR. 
85 
260 centims. from the mirror. The reflected beam was, however, intercepted by the 
telescope, whose object-glass, consisting of a spectacle lens of 10 centims. focal length, 
was 267 centims. from the mirror. The image was observed in the micrometer eye¬ 
piece. When the light was not too bright, and everything well adjusted, the images 
were exceedingly bright on a dark field, and each was the centre of a system of the 
most perfectly defined and fine interference fringes. By moving the lamp slightly, 
these fringes can be got to arrange themselves somewhat unsymmetrically about the 
luminous image, and thus enable an observer to distinguish a particular fringe with 
certainty ; one crossing the brightest part of the luminous image was always selected ; 
and if the fringes were unsymmetrical one was always much the most sharply marked. 
The similarity of the tracings of the dividing engine was such that each division had 
a set of fringes, as nearly as we could see identical with the fringes of the images on 
each side of it, so that a particular fringe could be selected as the fiducial mark of the 
imasfe, or the distance between two similar fringes could be taken as the distance 
between the millimetre divisions on the scale. At first the micrometer eye-piece was 
provided with a toothed or notched scale, but this not being sufficiently capable of 
sub-division, it was replaced by a scale divided on thin glass by a diamond, on the 
dividing engine, to '2 millim. A candle was then arranged so as to throw a suitable 
diffused light into the telescope, and, the micrometer scale presenting a diamond-cut 
edge, the coincidence of its divisions or otherwise with the interference fringes could 
be made out clearly and with great accuracy. The spider-line of the micrometer was 
not nearly so fine or so well defined as the interference fringes. By properly adjusting 
the fringes and illumination, everything became so distinct that it was much easier to 
read to micrometer scale divisions than it usually is to read with a lamp and scale 
to one division. We are quite sure that this method of illumination and scale reading 
will be found valuable by any one requiring to read small deflexions. The only points 
about which we would suggest caution are the following : — The light falling on the 
mirror must come from a source as small as possible, and a person setting up the 
arrangement for the first time would probably make the light much too bright and 
large for the interference fringes; the source must be a mere spark. The lens must 
be adjusted so as to be normal to the line joining the mirror and lamp, and this line 
must pass through its centre. The lens we used was an ordinary one, but it had 
good adjustments. The divisions on the Canada balsam or other varnish must be fine 
(we tried photographic transparencies, &c., instead, but they always looked woolly). 
The eye-piece of the telescope must be capable of very good adjustment. Ours 
was mounted on a fine tuning-fork stand by Konig ; it is no use unless the stand be 
very steady, and the adjustments, both vertically, horizontally, and in the altitude 
must be smooth and good. We made ours out of good brass tube without any diffi¬ 
culty. The source of light should be within reach of the observer at the telescope, 
in order that the final adjustment may be made by slightly moving the source, and 
by turning the lamp up and down till the best effect is obtained : this involves having 
