SCIENTIFIC SUMMAET. 
637 
what is termed the “BA Unit (1864) of Electrical Eesistance,” and 
shows some of the errors which have resulted from the employment 
of Dr. Siemens’ process. Dr. Siemens’ unit is known to electricians as 
the “mercury unit,” and it is defined as the resistance of a column of 
mercury, a metre long, and a square millimetre section at 0°. In reference to 
it. Dr. Matthiessen asserts — 1st, That no true mercury unit has been 
issued ; 2nd, That the units issued from time to time have not the same 
resistance. He proves his first assertion by showing that in making the 
calculations for the mercury unit, the wrong specific gravity of mercury was 
used, viz. 13‘557 at 0°, instead of 13’595 as found by Eegnault, Kopp, 
Balfour Stewart, and Neumann. In support of his second statement, Dr. 
Matthiessen alleges that there were two sets of coHs in the Exhibition of 
1862, said to be reproductions of the mercury unit. One was exhibited by 
Siemens of London, the other by Siemens of Berlin ; and when the two sets 
were compared with each other, they showed a difference of about 1*2 
per cent. — Vide Dr. Matthiessen’s paper in Philosophical Magazine, May. 
Pseudoscopic Vision through Prisms. — Some curious effects of prisms 
upon the apparent distance of objects viewed through them were pointed out 
by Mr. Dancer at a meeting of the Manchester Philosophical Society, which 
was held on April 4th. If we look with both eyes at an object, such as the 
flat top of a table for example, and then interpose a prism between one eye 
and the object, we discover, after a short time, that the portion of the surface 
to which the sight is particularly directed has apparently changed its distance. 
If, in trying the experiment, the edge of the prism is turned towards the 
nose, the flat surface will appear concave ; if, on the contrary, the base or 
thick edge is turned towards the nose, the surface will appear convex. The 
full effect of this alteration in the appearance of the object is not realized 
immediately ; some persons see it perfectly in a few seconds, others require 
some moments of steady gazing before it becomes evident to them. The 
character of the surface to which the vision is directed exercises some 
influence on the production of the effect. A circular table covered with 
a cloth of bright pattern, having a few articles disposed toward the edges, 
exhibits this fallacious vision in a marked degree. The angle of the prisms 
for performing these experiments should be about 15° ; if less than this, the 
alteration or depression of surface is not sufficient to produce a good effect ; 
if the angle is much greater than 15°, many persons are unable to unite th& 
refracted image of the prism with the real image seen by the other eye. 
Achromatic prisms are to be preferred to those that have not been corrected 
for colour. Experiments with these prisms have shown that the power of 
converging the optic axes differs very considerally in individuals. Oculists 
occasionally recommend prismatic lenses, mounted in spectacles, to assist 
persons whose recti interni muscles are not sufficiently powerful ; Mr. Dancer 
considers that it would be interestino' to know if those so assisted have 
o 
noticed the fallacious appearances which the healthy eye can appreciate. The 
pseudoscopic effects are exaggerated by using a prism for each eye, but in 
most persons this produces a painful sensation. He explains the phenomenon 
by supposing that in binocular vision we estimate the distance of an object 
by the degree of convergence of the optic axes. 
A Neiv Thermo- Element. — The following account of this piece of electrical 
