JULY 28, 1899. | 
ficiency of transparency which depends upon 
irregular reflexions 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 
of carbon and benzole, carefully adjusted so 
as to be of equal refractivity with the glass, 
the mass becomes optically homogene- 
ous, and therefore transparent. In conse- 
quence, however, of the different dispersive 
powers of the two substances, the adjust- 
ment is good for one part only of the spec- 
trum, other parts being scattered in trans- 
mission 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 colored. 
The color depends upon the precise com- 
position 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 of 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 accordingly at- 
tenuated light from the special source is 
then overlaid by diffused light of other 
colors. 
More precise determinations of the range 
of light transmitted were subsequently ef- 
fected with thinner strata of glass powder 
contained in cells formed of parallel glass. 
SCIENCE. 
105 
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 disturbance, 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 re- 
duce 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 refrangi- 
bility transmitted was estimated at 34 times 
the interval of the D’s. 
As regards the substitution of other trans- 
parent solid material for glass the choice 
is restricted by the presumed necessity. of 
avoiding appreciable double refraction. 
Common salt is singly refracting, but at- 
tempts to use it were not successful. 
Opaque patches always interfered. With 
the idea that these might be due to included 
mother liquor, the salt was heated to in- 
cipient redness, but with little advantage. 
Transparent rock-salt artificially broken 
may, however, be used with good effect, 
but there is some difficulty in preventing 
the approximately rectangular fragments 
from arranging themselves too closely. 
The principle of evanescent refraction 
may also be applied to the spectroscope. 
Some twenty years ago an instrument had 
been constructed upon this plan. Twelve 
90° prisms of Chance’s ‘ dense flint’ were 
cemented in a row upon a strip of glass, 
and the whole was immersed in a liquid 
mixture of bisulphide of carbon with a little 
benzole. The dispersive power of the liquid 
exceeds that of the solid, and the difference 
amounts to about three-quarters of the dis- 
persive power of Chance’s ‘extra dense 
flint.’ The resolving power of the latter 
glass is measured by the number of centi- 
