TRANSACTIONS OF SECTION A. 759 
_ than those obtained by the usual method of measuring with a bridge and one or 
two cells. Probably polarisation then interferes. 
Out of the five subjects tested four could stand no more than 18 volts alter- 
nating, with a maximum effective current of ‘03 ampére. The fifth took 54 volts 
and nearly a tenth of an ampére. 
7. On the Use of Fluor Spar in Optical Instruments. 
By Professor Sirvanus P. Tuompson, D.Sc. 
The author referred to the existing uses of fluor spar for experiments on radiant 
heat, and in the ‘apochromatic’ microscope lenses of Zeiss. The latter application 
derives its importance from the extremely low dispersion—relatively to the mean 
refractive power—of the material. To these applications the author now added 
that of the construction of spectroscopic direct-vision prisms; and he described 
two prisms, both constructed for him by Mr. C, D. Ahrens, one consisting of a 
fluor prism cemented between two flint glass prisms, and a second consisting of 
_ one Iceland spar prism cemented between two fluor prisms. The former was con- 
_ siderably shorter than the ordinary direct-vision prism of equal power. The latter 
_ had the property of polarising the light, as well as dispersing it, and presented the 
- novel feature of a true polari-spectroscope. 
_ 8. A new Direct-reading Photometer measuring from Unity to Infinity. 
5 By Freprrick H. Varuey. 
‘ __ This photometer was designed to meet the wants of electrical engineers and 
_ others. The conditions to be observed are that the instrument should be portable, 
_ have a range from one candle power to that of the electric arc, that the light to be 
+ measured and that of the standard should be exactly at the same distance from the 
sereen. This instrument consists of two discs each pierced by two semi-ring- 
shaped windows or apertures; these extend to half a circle (180°) ; both are of the 
same width (1 inch). The openings in the two discs are placed in reverse positions 
to one another, so that if one half-ring is opened to its full extent (180°) the other 
half-ring is entirely closed ; or, if the discs are shifted to an intermediate position, 
both apertures will be opened to an equal extent, namely, 90°. If in this position 
the discs are rotated it is obvious that an equal amount of light can pass through 
both rings; but, if the light to be measured is as one to seventeen candle-power, 
then the angular length of the two apertures must bear a proportionate ratio, in 
order that the two shadows shall be of equal density, and accordingly one aperture 
will be open to the extent of 10° for the brighter light, whilst that of the standard 
light is opened to 170°. Instead of dividing the circle into the usual 360°, the 
_ half-circle is divided into 2,000 parts, this giving a range from 1 to 1,999, or 2,000 
in round numbers. By still further shifting the discs this aperture may be entirely 
; closed, and read up toinfinity. The divisions of the half-circle are numbered from 
7 
* 
‘ 
left to right, and right to left, showing at once the fractional values of the angular 
extent of the opening, and thereby giving the value of the light. 
. In order to make the discs turn one upon the other, the author devised a modi- 
fication of the Ferguson paradox; that is to say, the discs are carried by 
independent shafts, one of which is hollow, to allow the central axis to turn 
within it; at the end furthest from the discs a cog-wheel is fixed to each axis.. By 
means of a sliding link the two wheels can be brought into gear with another axis, 
also provided with cog-wheels, each being of the same diameter, but one is cut 
with 100 teeth, whilst the other has 99 teeth ; thus, upon rotating the dises, each 
revolution of the gearing wheels advances the discs, and so changes the proportion 
_ of the openings, one decreasing whilst the other increases, and vice versd. This we 
ean do until the two shadows are of equal density, or approximately so. The final 
adjustment is then made by hand. Behind the windows two hollow cones are 
placed, which have their axis directed to a point common to both, but at some 
distance in front of the discs, where the two shadows fall upon the screen. A 
