536 
POPULAE SCIENCE EEVIEW. 
(6.) If the opinions of Lord Eosse and others, to the effect that these 
nebulae are systems formed by the aggregation of gaseous masses be correct, 
how is it possible that their permanence of general form can be maintained ? 
(7.) The opinion of the enormous distance of the nebulae from our system 
having been founded upon the supposed extent of remoteness at which stars 
of considerable brightness would cease to be separately visible in our tele- 
scopes, has no longer any foundation on which to rest, in reference at least 
to those nebulae which give a spectrum of bright lines. It may be that some 
of these are not more distant from us than the brighter stars. 
(8.) Mr. Huggins thinks that these nebulae are gaseous systems, possessing 
a structure and a purpose in relation to the universe, altogether distinct from 
the great cosmical masses to which the sun and fixed stars belong. He asks 
what is this special purpose, but at the same time he expresses the opinion 
that science will be more advanced by the laborious accumulation of facts 
than by the easier feat of throwing off brilliant speculations. 
The Mechanism of the Spectroscope. — In an article in a recent number of 
Sillimanh Journal, Mr. L. M. Eutherford gives an account of the apparatus 
for spectroscopic inquiry, which he has employed. In his instrument the 
two principal telescopes are provided with objectives of 1*6 inch aperture 
and 19 inches focal length. The slit, or collecting telescope, has but one 
motion about a vertical axis at the side of the platform and just in front of 
the objective, enabling it to command all parts of the platform. The 
observing telescope has two motions — one about the central axis of the 
instrument, and the other about a second vertical axis, which, by means of 
a slide capable of being clamped, can be placed under the last surface of any 
prism on the platform ; thus commanding by one motion the whole spectrum. 
Before the slit is a prism for the comparison of different spectra ; and the 
observing telescope is provided with eye-pieces of various powers. The first 
circuit consists of six prisms, which are of glass, faced with plates of glass 
cemented with glue and molasses. These are each about the angle of 60°, 
and present an aperture of 2*9 X 1*8 inch. The faces to receive the glass 
are carefully ground to a flat surface, and the glass, quite thick and free from 
veins, has been selected with reference to the flatness and parallelism of the 
sides. Since, however, it is scarcely possible to find glass with parallel 
surfaces, care has been taken so to place the glass, that the inclination of 
its faces is perpendicular to the axis of the prism. Mr. Eutherford gives a 
deal of further information touching the construction of the elements of the 
spectroscope, which we are compelled to omit notice of through want of 
space. Those of our readers who are interested in the subject should consult 
the original article in Silliman’s Journal, vol. xxxix.. No. 16. 
The Unit of Electrical Resistance. — The battle between Mr. Fleeming- 
Jenkin and Dr. Matthiessen on the one side, and Dr. Siemens on the other, 
upon this question has resulted in the complete triumph of the two former. 
Mr. Jenkin, who lately read a most valuable paper upon the subject of elec- 
trical units of resistance, proved incontestably that the method proposed by 
Dr. Siemens is unreliable. Since then. Dr. Matthiessen has pubhshed 
an essay upon the “ Specific Eesistance of the Metals ” (a subject which 
his numerous discoveries highly qualify him to undertake the investi- 
gation of), in which he advances many facts in support of the accuracy of 
