156 Scientific Correspondence. 
extremities of faint spectra, and render the observation of many lines , 
impossible, Be. 
Secchi’s spectroscope is on the whole much better me pe of the a 
former. The light of the great Roman equatorial passes through a slit 
at the focal point, then traverses a cylindrical Jens, then is pa par | 
allel by a lens, and then falls upon the prism, which is a compound strue- 
ture composed of four flint glass prisms of 90° cemented to five crown 
glass prisms of the same angles and. so ae age that the axis of the 
n 
amount to about the ante of two 65° prisms of flint ns iss. This di 
persion is so great that the lines in the stellar spectra are seen without an 
observing telescope. They are referred for measurement to an illuminated 
scale retiected from the last surface of the prism. This -scale is made by 
fine perforations in a metallic plate. The instrument is provided with 
a reflector for the purpose of comparing the star lines with those of @ 
any eniion i in Ke a of the s m. 
The advantage first named is mu os greater than would at first be sup: 
posed, as I have recently proved by the substitution of a bisulphid of 
Gine s ist for the flint glass formerly used in gas star s : 
With,an eye-piece of one half of the power forme rly oh 
spectrum longer than before with more than double the i intensity of light, 
and a consequent revelation of striz not before seen. 
I have perhaps trespassed too much upon deed space in this note upon 
the instrumental agency so far brought to bear pon the spectral anal 
of the stars. But these znvoatigndione are yet in a ihe cradle, and if, as 18 
hate they are destined to assume a great importance in the 
the constitution of the universe, it will not be amiss to point - bees 
about to embark in obs new field of abo the advantages and di 
the star spectroscope as it at present stands. é 
In the April No. of the Monthly Botioss, Mr. Glaisher describes 
servations on the length of the spectrum obtained from the sky at differ- 
ent a made by him during his balloon ascension on the _ 
h las - 
2 i 
are 
wing to want of oy (alt | 
han ; the s! oe at the time was of a p dark. blue, as tha 
is possible the light was inenflicient.”. - 
of the spectrum observed by. Mr. Glaisher ie 
serine cee bate and, to the monochromatic 
