SCIENCE. 
150 
SPECTROSCOPIC NOTES ON OBSERVATIONS— 
CHIEFLY SOLAR— 1879-80. 
By Prof. C. A. Young, of Princeton, N. J. 
(a) The magnesium lines of the b group and the sodium 
lines have been seen several times (first on June 5, t88o) 
doubly- reversed in the chromosphere spectrum — i. e. a bright 
line appeared as usual in the centre of the broad dark 
shade, and then this bright line widened and a thin dark 
line appeared in its centre. The phenomenon seems to be 
the exact correlative of the double-reversal of the bright 
sodium lines observable in the flame of a Bunsen burner 
under certain circumstances. 
(b) I have recently been able to repeat the observations 
on the H lines first made at Sherman in 1872. In the spec- 
trum of the chromosphere I find both Hi, and H 2 , (or K,as 
some call H 2 ) to be always reversed ; and what is more, II, 
is double , the principal line, which is in the centre of the 
dark shade, being accompanied by another of about half 
the strength, one division of Angstrom’s scale lower — i. e. 
less refrangible. Since last March I have always been able 
to observe the two H’s whenever I could see h, and H, 
invariably double. 
In the neighborhood of sun spots however, though both 
H anti K are usually reversed on the solar disc, H, is not 
double; its attendant line therefore belongs strictly to the 
spectrum of the chromosphere, and seems to be identical 
with No. 271 of my catalogue of chromosphere lines, 
though its wave length is about 3969 instead of 3970. 
The observations were made with grating of 17,280 lines 
to inch ; collimator and telescope 12-inch focus. 
(c) A high dispersion spectoscope has been constructed 
by combining the above-mentioned grating, having nearly 
four square inches of ruled surface, with collimator and 
telescope of 3 inches aperture and 42 inches focus, the 
magnifying power employed varying from 50 to 200. The 
apparatus is strapped to the tube of the equatorial, and 
thus kept directed to the sun, an image of which is formed 
on the slit by an anachromatic lens of 3 inches aperture. 
The performance of the grating is admirable when per- 
fectly flat — a force of oz. applied at one corner is how- 
ever sufficient to distort the plate (of speculum metal) 
inch thick by about 3j£ inches square, to an extent which 
seriously impairs the definition ; it is sensitive to such dis- 
tortions to a degree entirely unexpected. This instrument 
doubles an enormous number of the Fraunhofer lines. 
Out of 47 lines between C and G marked by Thalen as 
common to the spectra of two or more bodies, 38 are double 
or triple, 3 are doubtful (from difficulty of identification), 
and 6 only are single so far as the instrument can show, 
(d) Distortion of solar prominences by a diffraction 
spectroscope. Generally, in such an instrument, the forms 
seen through the opened slit are either disproportionately 
extended or compressed along the line of dispersion. If 
the angle between the normal to the grating and the view- 
telescope is less than that between the normal and the 
collimator, there will be compression or flattening, and 
vice versa. The mathematical investigation is very sim- 
ple — 
Let n be the order of the spectrum observed. 
Let A be the order of the wave length of the ray. 
Let S be the distance between adjacent lines of grating. 
Let t be the angle between normal to grating and tele- 
scope. 
Let k be the angle between normal to grating and collima- 
tor, and finally n=r+K=angle between telescope and 
collimator, which is supposed constant. Then from the 
fundamental conditions of spectrum formation n/f=S (Sin_ 
r — Sin. k) or Sin. r= n J^ 4- Sin. k, whence dr= 9 ? s ' ds or 
S Cos. T 
(Cos. rt+Sin. a Tan. t) d/c, whence, in general, dr will not 
equal d«. 
Special cases — 
1. If K=r, there is no distortion — but also no dispersion ; 
it is the case of simple reflection. 
2. If k= o, grating being kept normal to the collimator, 
then £=Sec. « d*. 
3. If t—o, grating being kept normal to the telescope 
and moving with it, then dr=Cos. a d«. 
4. If a—go° dr=Tan. r d/(. 
5. If a=o. dr=d/> and there is no distortion. This is 
possible only by using the same tube both for collimator 
and view-telescope, the grating being slightly inclined. 
The principal difficulty with this form of instrument lies in 
the reflections from the surface of the object glass, which, 
it is hoped, may be avoided by a special construction of 
the lens. An instrument on this plan is in process of con- 
struction by the Clarks, for the Physical Laboratory at 
Princeton, and nearly completed. 
ON THE THERMO-ELECTRIC ELECTRO-MO- 
TIVE POWER OF FE. AND PT. IN VACUO. 
By Prof. C. A. Young, of Princeton, N. J. 
Eisner, a few months ago, published a paper asserting 
that the thermo-electric power of Antimony and Bismuth 
is destroyed by removing them from all contact with oxy- 
gen, and inserting them in an atmosphere of pure nitrogen. 
From this he argues that the thermo-electric force in gen- 
eral is due to the contact of the gases which bathe the 
metals. The following experiment was tried to test the 
theory. 
By the kindness of Mr. Edison and Mr. Upton a vacuum 
tube was prepared in Mr. Edison’s laboratory, containing 
an iron wire, about 2 inches long, firmly joined to two 
platinum terminals which passed through the walls of the 
tube ; the tube was exhausted until a 2-inch induction coil 
spark would not pass ^ of an inch in the gauge-tube, indi- 
cating a residual atmosphere of about one-millionth. The 
wire was heated too in candescence during the exhaustion, 
in order to drive off any possible occluded gases. The 
platinum wires outside the tube were joined to iron wires, 
the joinings being covered by glass tubes slipped over 
them, and a sensitive reflecting galvanometer was included 
in the circuit. By laying the tube and connected joinings 
in the sunshine, and alternately shading one or several of 
the joinings, it was found that the electro motive power of 
the joinings within the tube was precisely the same as that 
of those without, and the development of current just as 
rapid. There was no trace of any modification due to the 
exhaustion. 
ON THE ABSOLUTE INVISIBILITY OF ATOMS 
AND MOLECULES. 
By Prof. A. E. Dolbear. 
Maxwell gives the diameter of an atom of hydrogen to be 
such that two millions of them in a row would measure a 
millimeter, but under ordinary physical conditions most 
atoms are combined with other atoms to form molecules, 
and such combinations are of all degrees of complexify ; 
thus a molecule of water contains three atoms, a molecule 
of alum about one hundred, while a molecule of albumen, 
according to Mulder, contains nine hundred atoms, and 
there is no reason to suppose albumen to be the most com- 
plex of all molecular compouuds. When atoms are thus 
combined it is fair to assume that they are arranged in the 
three dimensions of space, and that the diameter of the 
molecule will be approximately as the cube root of the 
number of atoms it contains, so that a molecule of alum 
will be equal to 
( V100 — d 6 a) — ■ - — mm. 
V 100 4.04) 2000000 431000 
and a molecule containing a thousand atoms will have 
a diameter of - 2 - nsWotr = TuoToV mm - Now a good micro- 
scope, will enable a skilled observer to identify an object so 
small as the - 4 - u Vp mm. Beale in his works on the micro- 
scope pictures some fungi as minute as that, and Nobert’s 
test bands and the markings upon the Amphiphtra peluc- 
ida, which are of about the same degree of fineness, are easily 
resolved by good lenses. If thus the efficiency of the mi- 
croscope could be increased fifty times ( ifl 4 fl u fi i< !L!l =5 0 ) >t 
