64 NATURE 
ROTATION PERIop OF Mars.—Mr. W. F. Denning has 
recently secured some measures of the times of transit of the 
Syrtis Major (Kayser Sea), which in conjunction with observ- 
ations made by him in 1884 and 1869 give a critical value for 
the period (Odservatory, May 1899, p. 195). On February 4, 
1869, the Syrtis Major was in mid-transit at 11h., while on 
February 14, 1884, when Mars was similarly situated with 
reference to the Earth, the transit occurred at 5h. 55m. Other 
transits were taken as February 15, 6h. 35m. ; February 19, 
gh. 5m.; February 22, 11h. 4m. Now, after another interval 
of fifteen years, the transit on March 7, 1899, occurred at 
8h. 31m. The whole period between 1869 February 4, Ith. 
and 1899 March 7, 8h. 31m., comprises 10,987 days, 21 hours, 
31 minutes, during which Mars has performed 10,710 rotations. 
The mean period during this interval thus becomes 
24h. 37m. 22°70s. 
This value is intermediate between those of Proctor and 
Bakhuyzen. 
AN IMPROVED RESISTANCE-BOX. 
MESSRS. GAMBRELL BROS. have recently introduced 
a resistance-box of improved design, which gives promise 
of eliminating several of the disadvantages of the usual post- 
office pattern. Fig. 1 shows the appearance of the box with the 
cover removed to show the working parts. The coils, which 
hang vertically in the lower part of the box, are brought up to 
Fic. 1.—General view of the box, from above, showing the numbered slide 
rods, the contact shoes and the terminal studs of the coils. The handle 
(H) at the right is for clamping all the contact shoes simultaneously. 
terminal studs (T) seen in Fig. 2, arranged in five rows, one 
of which, in two sections, forms the ‘‘ ratios” used as two arms 
of the bridge. The upper surfaces of these studs are semi- 
circular, fitting the concave surfaces of the sliding contact shoes 
(s). The four rows other than the ‘‘ ratios” provided for thou- 
sands, hundreds, tens and units, reckoned from the side nearest 
the terminals and key. Each of these contact shoes slides with 
slight friction on a brass bar running the length of the box, and 
supported at each end by metal pillars held down by springs in- 
side the box. To move the shoes from one stud to another other 
Fic. 2.—Showing construction of slider, spring contact bar, &c. 
brass rods are attached, which slide through ebonite bushes on 
the end of the box. On these rods are engraved the figures 
giving the amount of resistance in use, the value of any par- 
ticular resistance in circuit being indicated by the number show- 
ing just outside the ebonite bush. To ensure the contact shoe 
being properly fixed on the studs, a spring detent (p, Fig. 2) is 
provided under each bar, so that the resistances may be changed 
without the experimenter needing to watch the bar. All the 
bars being arranged to give the resistance required, it will be 
evident that its total amount can be read straight off at the end 
NO. 1542, VOL. 60] 
[May 18, 1899 
of the box, being given by the row of figures close to the four 
ebonite bushes. For example, the reading of the resistance in 
circuit, as shown in Fig. 1, is 2310. This is itself a great con- 
venience, and will prevent any chance error in adding. Asan 
additional help to maintaining the contacts as constant and 
perfect as possible, when the proper resistance has been found, 
all the four shoes are drawn tightly down on to the terminal 
studs by turning the handle H, seen under the ends of the rods 
in Fig. 1. This actuates a cam inside, which moves the small 
pillars at each end of the brass bars passing through the contact 
shoes. At the same time, the arrangement acts as a clamp, so 
that while the handle is turned the resistances cannot be changed, 
All the pillars are held down by springs, so that when not 
clamped by the handle H the sliding to and fro is accompanied 
by sufficient friction to keep the contact surfaces clean. 
In consequence of the ingenious method adopted for reading 
off the figures, rendering access to the contacts themselves quite 
unnecessary, the whole of the system of studs and sliding bars is 
covered in permanently, so that they and the ebonite insulating 
block are kept free from dust and corrosion. The studs, being a 
considerable distance apart, should permit of a very high in- 
sulation resistance, while at the same time allowing a large 
surface contact between the shoe and the stud. 
It will be seen that this new form of box has many advantages 
to recommend it to notice. The simplicity and rapidity of read- 
ing, its compactness, and its non-liability to deterioration, should 
cause it to find favour both in laboratory and testing-room 
experience. 
TRANSPARENCY AND OPACITY} 
ONE kind of opacity is due to absorption ; but the lecture 
dealt rather with that deficiency of transparency which 
depends upon irregular reflections and refractions. One of the 
best examples is that met with in Christiansen’s experiment. 
Powdered glass, all from one piece and free from dirt, is placed 
in a bottle with parallel flat sides. In this state it is quite 
opaque ; but if the interstices between the fragments are filled 
up with a liquid mixture of bisulphide of carbon and benzole, 
carefully adjusted so as to be of equal refractivity with the 
glass, the mass becomes optically homogeneous, and_ therefore 
transparent. In consequence, however, of the different dis- 
persive powers of the two substances, the adjustment is good 
for one part only of the spectrum, other parts being scattered 
in transmission much as if no liquid were employed, though, of 
course, in a less degree. The consequence is that a small source 
of light, backed preferably by a dark ground, is seen in its 
natural outlines but strongly coloured. The colour depends 
upon the precise composition of the liquid, and further varies 
with the temperature, a few degrees of warmth sufficing to 
cause a transition from red through yellow to green. 
The lecturer had long been aware that the light regularly 
transmitted through a stratum from 15 to 20 mm. thick was of 
a high degree of purity, but it was only recently that he found to 
his astonishment, as the result of a more particular observation, 
that the range of refrangibility included was but two and a half 
times that embraced by the two D-lines. The poverty of general 
effect, when the darkness of the background is not attended to, 
was thus explained, for the highly monochromatic and accord- 
ingly attenuated light from the special source is then overlaid by 
diffused light of other colours. 
More precise determinations of the range of light transmitted 
were subsequently effected with thinner strata of glass powder 
contained in cells formed of parallel glass. The cell may be 
placed between the prisms of the spectroscope and the object- 
glass of the collimator. With the above-mentioned liquids a 
stratum 5 mm. thick transmitted, without appreciable disturb- 
ance, a range of the spectrum measured by 11°3 times the 
interval of the D’s. In another cell of the same thickness 
an effort was made to reduce the difference of dispersive 
powers. To this end the powder was of plate glass and the 
liquid oil of cedar-wood adjusted with a little bisulphide of 
carbon. The general transparency of this cell was the highest 
yet observed. When it was tested upon the spectrum, the 
range of refrangibility transmitted was estimated at thirty-four 
times the interval of the D’s. 
As regards the substitution of other transparent solid materia) 
1 A discourse delivered at the Royal Institution on Friday, Mareh 24, by 
the Right Hon. Lord Rayleigh, F.R.S. 
