SCIENTIFIC SUMMARY. 
81 
out of any one of the prisms of either battery, there is no waste of light in 
the passage through the intermediate prism. The two batteries are auto- 
matically adjusted to minimum deviation for all rays. After passing 
through the two batteries, the light is reflected and sent again through both 
batteries, and by means of a diagonal prism the observer is enabled to ex- 
amine the spectrum through an eye-tube at right angles to the axis of the 
collimator. In this last respect the plan differs in nothing from that 
adopted long since by Janssen and others. The novelty of the instrument 
consists in the arrangement for including a second battery in the automatic 
system. The dispersive power thus secured — with perfect adjustment — • 
can be made as great as that due to twenty-one or even twenty-three 
equilateral prisms of dense flint-glass. As motion is communicated to the 
intermediate battery, the effective length of connection is no greater than in 
the case of the single automatic battery. 
Grubb's Automatic Spectroscope. — Mr. Grubb has devised an automatic 
spectroscope, which is intended for use with the fine telescope lately 
placed by the Royal Society under the management of Dr. Huggins. In 
this instrument, which was devised so far back as last March, there are five 
compound prisms ; each consisting of a rectangular prism of dense flint- 
glass, bounded by two prisms of crown-glass of small refracting angle, and 
introduced merely to get the light through the rectangular prism. These 
five prisms are automatically adjusted to minimum deviation, and the light 
is taken twice through the battery and viewed through a fixed eye-tube, as 
in Mr. Proctor’s plan. Whether securing minimum deviation through a 
compound prism of this sort is equivalent to securing those optical condi- 
tions which result in the case of minimum deviation through a single prism, 
is a question which Mr. Grubb has not, so far as we know, considered. It 
seems unlikely that the spectral lines can be so well defined when these triple 
prisms are used as where single prisms are employed. But experience only 
can determine which plan is preferable. 
The Total Eclipse of the Moon last July . — The Astronomer Royal observed 
this eclipse in a peculiar manner. 11 Being shortsighted,” he says, l( (the 
distance for distinct vision less than five inches), and every luminous object 
being seen as a broad blur, 1 can compare the quantity of light from 
sources of very different character. Thus I found that the quantity of light 
received from the eclipsed moon was less than that from Saturn and greater 
than that from a Aquilae, including some from (5 and y Aquilae. The light 
of the moon increased considerably in ten minutes after the time of central 
eclipse.” “I infer,” he adds, “ that the only part in which the shadow is 
nearly total is the centre of the shadow, and that a large amount of light 
falls within the geometrical limits of the true shadow.” 
The Satellites of Uranus . — A question has been raised as to the existence 
of any more satellites of Uranus than the four seen by Mr. Lassell. Professor 
Pritchard has indeed somewhat roundly asserted that to believe in more than 
these four is to attack Mr. Lassell. Very few seem to be aware how the 
matter really stands. Mr. Proctor, in the Monthly Notices of the Astro- 
nomical Society, defends the claims of Sir William Herschel’s other four 
satellites. He points out that Sir William Herschel by no means expressed 
(as the Savilian Professor supposes) a mere suspicion of the existence of these 
YOL. X. — NO. XXXYIII. G 
