374 
POPULAR SCIENCE REVIEW. 
A New form of Electro Magnet was exhibited to the Physical Society 
by Dr. Stone. It was wound with best charcoal annealed iron wire of 
5 millim. section, in four parallel circuits. Each pole was cast after 
winding into a solid block of paraffin, and turned in a lathe. It was 
expected that the latter device would increase the inductive effect of the 
spirals, and it appeared that the lifting power was somewhat augmented. 
The cores had originally been wound with large copper-wire, of the same 
weight, in three parallel circuits. The lifting power for moderate batteries, 
of from five to six Bunsen’s cells, had been increased fourfold by the sub- 
stitution of the iron. The object was to produce a large diffused magnetic 
field for the purpose of physiological experiments on anaesthesia and other 
nervous diseases, in which there seemed to be ground for the belief that the 
effects of magnetism were far from inappreciable. 
The Resolving Power of Telescopes , in its experimental point of view, is 
examined by Lord Rayleigh. He states that the only work on the subject 
he is acquainted with is that of Foucault, who examined the resolving 
power of a telescope of 10 centim. aperture on a distant scale lighted by 
direct sunshine. 
The object viewed in these experiments was a grating of fine wires formed 
by screwing the ends of a stout brass wire, bending it into a horse-shoe, and 
winding wire of half the pitch diameter into the grooves of the screw. The 
whole is fixed with solder, and the wires on one side are cut away. For 
rough purposes, common wire gauze of 80 to 40 meshes per inch answers. 
The grating was backed by a soda flame, though a common paraffin lamp 
may be substituted. 
The telescope was provided with a cap to which various diaphragms can 
be fitted over the object-glass. These may be circular or rectangular, 
the latter being placed with its long axis parallel to the wires of the 
grating. 
The observation consisted in finding the greatest distance at which the 
wires can be seen apart. This proved to be more definite than might have 
been expected, not differing more than 2 or 3 per cent for various observers. 
Two slits, half an inch long, and of T07 and T96 inch wide, were used. 
The width was taken by a wedge and measured by callipers. The distances 
were 91 ’5 and 168'5 inches. These corresponded to angular intervals be- 
tween consecutive lines of and According to theory, the mini- 
mum angle is about that subtended by the wave-length of light, \, at a 
distance equal to the width of the slit a. In this case \ = 5*89 x 10 — 5 
centim. and a = '107 x 2-54 or 196 x 2 # 54 centim. Thus: - = 
a 
ttItv or rfrs) agreeing closely with the observations. Circular apertures 
were also used with less satisfactory results. To have equal resolving 
power, the circular aperture must be about a tenth part wider than the 
slit. 
To show the dependence of resolving power on aperture, it is sufficient to 
look at wire-gauze backed by the sky or by a flame, through a piece of card- 
board pierced by a needle and held close to the eye. By varying the distance 
a point is easily found at which resolution ceases ; but the telescope allows 
the use of a wider, and therefore more easily measurable aperture. 
