SCIENTIFIC SUMMARY. 
101 
in minute velvety black crystals, appearing yellow by transmitted light. It 
is also met with compact, opaque, and purplish brown. Its lustre is resinous ; 
its streak yellow ; H. = 3 j sp. gr. 5*894. The analysis leads to the formula 
(PbCu) 3 Vn 2 0 8 + 2H 2 (PbCu)0 2 , in this respect analogous to erinite and 
dihydrite. 
Professor Roscoe has also made an examination of Roscoelite (see " Pop. 
Sci. Rev.” Oct. 1876, p. 431), and his analysis leads him to ascribe to it 
quite a different constitution from that given by Dr. Geuth . 
PHYSICS. 
Law of Refraction. — Professor Foster, at a recent meeting of the Physical 
Society, exhibited and described an instrument for illustrating the law of 
refraction. It is founded on the well-known method of determining the 
direction of the ray after refraction, by means of two circles described from 
the point of incidence as a centre, the ratio of whose radii is the index of 
refraction. If the incident ray be projected to meet the inner circle, and 
through the point of intersection a vertical line be drawn, the line drawn 
from the point of incidence to the point where this meets the outer circle is 
the direction after refraction. This principle is applied in making a self- 
adjusting apparatus as follows: — A rod representing the incident ray is 
pivoted at the point of incidence, and projects to a point about four inches 
beyond. To its extremity is attached a vertical rod, which slides through a 
nut in another rod, also pivoted at the point of incidence. The lower ex- 
tremity of the vertical rod is attached to a link, so fixed as to constrain it 
to remain vertical. By this means the two rods always represent respec- 
tively the incident and refracted rays, and the index of refraction can be 
varied by altering the position of the nut through which the vertical rod 
passes, on the rod to which it is attached. 
PHYSIOLOGY. 
Vaso-motor Centres in the Cortex Cerebri. — MM. Eulenburg and Landois 
(“ Central blatt fiir die med. Wiss.,” April 1876) have, according to the 
“ Academy,” discovered that the destruction of certain limited portions of 
the cortical substance in one hemisphere is followed by relaxation of the 
arterioles in the limbs on the opposite side of the body. This vaso-motor 
paralysis causes a rise of temperature amounting in some cases to 5°-7° C. 
in others to l|- 0 -2 0 C. only, and lasting from one day to several weeks. 
Stimulation of the same portions of the cortex by induced currents is followed 
by a slight and transient depression of temperature in the opposite extremi- 
ties (*2° to *6° C). The associated musculo-motor disturbances showed that 
these thermic or vaso-motor centres are situated in close proximity to the 
corresponding motor centres of the limbs. A successful attempt was made 
to differentiate the vaso-motor centre of the fore from that of the hind leg 
the former was found to lie a little to the front and to the outer side of the 
latter. 
