158 Chronicles of Science. | Jan. 
of the red lithium line is 0:0006733 millimetres. The wave length 
of the yellow sodium line is 0:0005918 millim ; * that of the green 
thallium line is 0°:0005348 millim, whilst that of the blue strontium 
line is 0:0004631 millim. 
Perhaps the most powerful spectroscope in the world has recently 
been constructed by Professor Cooke. It has nine bisulphide of carbon 
prisms, which are constructed of cast-iron, with parallel sides of glass, 
special precautions being taken to remedy the curvature of the glass 
plate from the hardening of the glue. The nine prisms are almost 
optically perfect, and the light is bent by them through nearly 360°. 
By its means Professor Cooke has established the following points :— 
1. The lines of the solar spectrum are as innumerable as the stars ; 
at least ten times as many being visible as are shown in Kirchhofi’s 
Chart, with an infinitude of nebulous bands, just on the point of being 
resolved. No less than nine additional lines are seen enclosed within 
the fixed line D, one being nebulous and showing signs of resolva- 
bility under further increase of power. 2. It proves that the coinci- 
dences between the metallic lines of artificial spectra and the corres- 
ponding dark lines of the solar spectrum remain perfect under this 
increase of optical power. The two sodium lines can be spread out 
so as to allow of the thousandth part of the intermediate space being 
distinguished, and still their coincidence with the Fraiimhofer lines is 
absolute. 38. Many of the bands of metallic spectra are broad coloured 
spaces crossed by bright lines; this is especially the case in the calcium, 
barium, and strontium spectra. 
Some reliable experiments on the photometric value of the electrie 
light have been published by Professor W. B. Rogers.t The battery was 
very powerful, consisting of 250 carbon elements, each having an active 
zine surface of 85 square inches. They were grouped in fine battalions 
of 50 each, and the light was obtained in an apartment where a range 
of about 50 feet could be obtained for the photometric apparatus. In- 
stead of an ordinary standard light, equivalent to 20 candles, a unit 
was substituted ten times as great, equal therefore to 200 candles. By 
a series of experiments with the naked electric light unaided by a re- 
flector, it was found that its intensity was from 52 to 61 times as great 
as the standard light, making it equal in illuminating power from 10,000 
to 12,000 standard sperm candles. With the rays concentrated by a 
parabolic reflector, its illuminating force had a value equal to several 
millions of candles all pouring forth their light at the same time. The 
only previous measurement of the illuminating power of the electric 
light which we can call to mind is one given by Bunsen. This was 
taken with a less powerful battery (48 cells), and the photometric 
equivalent was estimated at 572 candles; giving a proportion of 12 
candles to the cell, whilst Professor Rogers’ estimate gives the ratio 
of 40 candles to the cell. 
* Fraiinhofer’s measurement for the dark line D of the solar spectrum gave it 
a wave leneth of 0:0005888. 
+ ‘Silliman’s Journal,’ vol. xxxvi. p. 307. 
