150 



SCIENCE. 



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) 

 a^H^/j'-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, Hi 

 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 Hi 

 invariably double. 



In the neighborhood of sun spots however, though both 

 H and K are usually reversed on the solar disc, Hi 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) y% 

 inch thick by about 2> l A 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, 



(</) 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 f be the order of the wave length of the ray. 



Let S be the distance between adjacent lines qf grating. 



Let r be the angle between normal to grating and tele- 

 scope. 



Let k be the angle between normal to grating and collima- 

 tor, and finally « = r-|-K = angle between telescope and 

 collimator, which is supposed constant. Then from the 

 fundamental conditions of spectrum formation n/— S (Sin 



t — Sin. *)or Sin. 7= -fSin. it, whence d~ = ' d'>, or 



S Cos. r 



(Cos. a + Sin. a Tan. r) d«, whence, in general, dr will not 

 equal d«c. 



Special cases — 



1. If Krrr, there is no distortion — but also no dispersion ; 

 it is the case of simple reflection. 



2. If k=0, grating being kept normal to the collimator, 

 then i Sec. a d*. 



3. If r=o, grating being kept normal to the telescope 

 and moving with it, then dr-=Cos. a d/>. 



4. If a=go° dr=Tan. r dx. 



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 complexity ; 

 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 



(Vioo = 4.64) 4 " 4 = - — - — mm - 



\ iuu 4..^/ 20O0O00 4310OO 



and a molecule containing a thousand atoms will have 

 a diameter ol . V',,,,,, nn nnrc mm - Now a good micro- 

 scope, will enable a skilled observer to identify an object so 

 small as the .,,,',,„ 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 Amphiplura 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 ( iU 4 U u'« ua = 5o) it 



