308 
POPULAR SCIENCE REVIEW. 
through a single or double battery of prisms (single or compound) can readily 
be extended to cause the light to pass four, or even six or eight times through 
a battery. It is only necessary that the prisms, already doubled in height, 
that the light may pass twice through them, should be made four, six, or 
eight times as high as in an ordinary battery, rectangular prisms, precisely 
like the one cemented to the last half-prism in the return battery, being 
provided to raise the light rays, story by story, so to speak, as they pass 
backwards and forwards through a battery of these tall prisms. We be- 
lieve Messrs. Grubb have proposed to make for Dr. Huggins’ telescope a 
four-storied battery of compound prisms ; while we learn that in America 
Professor Young and Dr. Langley are having instruments constructed on 
this plan. 
We note here that Mr. Browning claims priority of Messrs. Grubb as 
respects the construction and employment of compound prisms. He remarks, 
that since July 1869, he “has made several compound prisms of very dense 
flint glass and light crown glass for the present Earl of Rosse.” He is now 
constructing a compound prism formed of three crown and two flint prisms 
for Mr. Lewis Rutherford, of New York. It is not a direct vision prism, 
as might be supposed from this description, the two outermost crown prisms 
being of small refracting angle. Mr. Browning remarks that the reason he 
has not used compound prisms more frequently is that such prisms are more 
expensive than ordinary ones, even allowing for the extra dispersive power 
obtained, and that, “ in consequence of the minimum angle of deviation 
of compound prisms being greater ” (sic, but the word should obviously 
have been less), and their length greater, the size of spectroscopes would 
require to be increased, and they would thus be rendered more cumbersome 
as well as more expensive.” 
The Total Eclipse of the Sun on December 11, 1871. — Attention is already 
beginning to be directed to this eclipse. It seems unlikely that any ex- 
peditions will be sent out from England to observe it ; but a3 the track of 
totality passes over parts of India, Ceylon, and Northern Australia, it is 
probable that useful observations will be made. In India especially it is 
likely that skilful observers may have an opportunity of studying the eclipse, 
since the northern boundary of the track of totality lies but a short distance 
south of Madras, where there is an excellent observatory, under the manage- 
ment of Mr. N. Pogson. The following account of the most important 
features of the eclipse is extracted from a paper by Mr. Ragoonathachary, 
communicated by Mr. Pogson to the Royal Astronomical Society : — 11 The 
central line of the eclipse will first meet the Earth’s surface in the Arabian 
Sea, and, entering on the western coast of India, will pass right across one of 
the most important parts of Hindustan in a S.E. by E. direction. In this 
part of the peninsula the Sun will be about 20° above the horizon when 
totally obscured. The duration of totality will be two minutes and a 
quarter and the breadth of the shadow about 70 miles. On leaving the 
eastern coast of the Madras Presidency the central line will cross Palk’s 
Straits, passing about 10 miles S. W. of the island J affnapatam, and over 
the northern part of Ceylon, where the small towns of Moeletivoe and 
Kokelay will lie near the central line ; and also the well-known naval station 
of Trincomalee, which will be about 15 miles S.W. of the line. Con- 
