Addendum on Linear Dispersion of the Spectrum  (from Early Solar Physics)


                              EARLY SOLAR PHYSICS,  A. J. Meadows, 1970 (Prepared by Fredrick N. Veio, Sept. 10, 2003)



I bought the book shortly after publication, for it gives an insight of visual observations of skilled solar workers using large prisms about 1860 and  using gratings several  years later.. The Zeeman effect was visually seen but at the time they did not know what was causing the widening of some photospheric lines in the penumbra and in the umbra of a sun spot. The book is a collection of solar papers written by P. A. Secchi, C. A. Young,  J. N. Lockyer, G. E. Hale and others before and at the turn of the century of 1900. Many good drawings are presented too.


The principle of a magnetic field showing the  Zeeman effect was discovered by P. Zeeman, a Dutchman, in 1896 in his earthly laboratory. Magnetic fields in sun spots were discoverd by  G. E. Hale in 1906, using the 60-foot solar tower on Mt. Wilson, published in  1908. Linear dispersion 0.7A/mm second order. The horizontal Snow telescope was ready in 1904, and the 150-foot solar tower working in 1912.


Most photospheric lines, dark visual core of 0.03A, are not sensitive to the Zeeman effect. Some lines are slightly sensitive, widening to 0.06A, not easy to see, and a few here and there are very sensitive, widen to 0.3A, very easy to see.


I make quotations and give the page number. In parenthesis are inserted words for clarification. This presentation is in no way final, just a convenient set of events to read.


Now for the quotations from the various pages.



Page 21. First detailed map of the solar spectrum produced by Kirchhoff in 1861. Used four large prisms, dispersion was not linear per wavelength per unit length on a scale.


Page 22. About 1864 Angstrom produced a new map of the solar spectrum, employing a reflection grating, giving normal linear dispersion per wavelength on a scale. Grating had defects, gave ghosts.


High quality reflection gratings were not available until about 1880 by Rowland of Princeton University. Excellent qualilty replica grating produced about 1957 by Bausch and Lomb in the USA.The cost was about one-tenth of the original. For example in 1964, a replica of 32x30mm area with 1200 gr/mm cost $96. The original grating cost was about $1000.


Page 22. Rowland started a new solar spectrum map by photography in the 1890s. Used a very fine grating. Maped solar lines from 3000A to 7000A wavelengh. Concave grating had 800 gr/mm, 21 feet focal length (about six meters), about 2A/mm in the first order. Prints from the glass plates were enlarged about seven times, giving 0.3A/mm on the solar map as viewed in your hands. All the measured 20,000 spectral lines were published in the Astrophysical Journal prior to 1900.


Page 28. In 1876 Young, USA, used the first astronomical spectroscope with a grating.


Page 38. In 1866 Lockyer began to study the spectra of sun spots. Used large prisms with a base about 80mm long.  "Some (lines) were broader and darker in spots." About this time Secchi was examining sun spot spectra.


Page 39. In 1883 Young showed "the spot (spectrum) under high dispersion (about 3A/mm) consisted of very many fine absorption lines close together.....These fine lines had a spindle-shape (widening due to the Zeeman effect)."


Page 40. In the 1880s Lockyer found several (molecular) bands in the spectra of sun spots.


Page 71. About 1880 "dry emulsions replaced the wet collodion process."


Page 71. Deslandres and Hale,1869,  independently constructed a spectroheliograph, two related but different versions . Page 237, Janssen suggested the shgraph in 1869. Hale invented the spectrohelioscope about 1927.


Page 121. In 1871 Vogel used a 11 inch refractor and grating.


Page 123. May of 1871, Vogel saw a large sun spot, "all the (photospheric) lines in the spectrum were involved in this (spectral) displacement."  Magnetic lines in a spot sometimes have a shear region, and as the magnetic lines move to straighten out, a "velocity of 4 to 5 km/sec" will be detected (need high dispersion, about 3/mm). A nice related article in Solar Physics by  H. Zirin, 1973, vol. 32, page 173, shows excellent photos of the photospheric spectral displacement, also mentions the velocity of 5 km/sec too.


25.4mm equal one inch. One meter equals 39.4 inches.


Page 125. In 1869 Young used a solar spectroscope designed with the telescope section of 4 inch (100mm) achromat with 30 inch f.l. (750mm) and 7X Huygenian eyepiece projection for a 2.2 inch (55mm) sun image (gives 210 inch equivalent f.l., or almost 5.3 meters) on the entrance slit. The spectroscope section had two 16 inch f.l. (400mm) achromats and five prisms of 2.2 inch (55mm) height  with a base of  3.2 inch (81mm). The spectrum of 45 inch  length (1125mm) from the violet to the red, average dispersion about 4A/mm in the green. The Ni line between the yellow D 1 and D 2 was "perfectly distinct."


Page 174. In 1866 Locker used a direct vision spectroscope of five prisms on a 6.2 inch (155mm) refractor of F 15. Photospheric lines "appeared thicker when they crossed the spot spectrum."


Page 177. In 1883 Young, Princeton University, used a 23 inch (575mm) refractor of F 15, 29 feet f.l. (8.3 meters), gives about a 3.2 inch (81mm) sun image. Had a 600 gr/mm grating of 3.2 inch by 5 inch (81mm  x 126mm). Spectroscope was one achromat, five inch (125mm) diameter and 48 inch f.l. (1200mm), Littrow design. Linear dispersion about 13A/mm in first order, 6A/mm in the second order and 4A/mm in the third order.


Not quoted from the Meadow book, but Mitchell used the same 23 inch (575mm) refractor to observe visually 680 spot lines from H beta to "a" atm line (blue, yellow, orange-red). Published in Astrophysical Journal, 1905. Had a large grating 800gr/mm, 30 inch f.l. (760mm) spec optics, linear dispersion about 5A/mm in the third order.


Page 179. Young, "Resolution of the spot spectrum into.....fine lines is most easily made out in the green and blue." "In the region....between b 1 and b 2 (green Mg lines, 0.1A core) the single (photospheric) lines appear (0.03A dark visual core, Veio) to be each about half as wide as the components of b 3 (Fe, 0.05A dark core, Veio)."  "(lines)are a little wider in the middle of the spot spectrum, in fact spindle-shaped, running out into extreemely fine threads where they pass into the penumbra." 


In 1964 Veio measured the dark visual cores of spectral lines with his spectrohelioscope in the spec mode, linear disperson 4A/mm first order and 1.6A/mm second order  green. Strong chromospheric lines are wide. Violet K line has a dark core and wide wings of 3A; violet H line, 2A dark core. H alpha is 0.6A, and H beta is 0.4A. Medium strong lines are the two sodium lines, about 0.1A. The two Mg lines of b 1 and b 2 have 0.1A dark core. The Fe b 3 and the Mg b4 are 0.05A. Less strong photospheric lines are 0.03A dark core. Faint photospheric lines are  0.015A.


Page 179. Conspicuous bright line at 5162.3A wavelength in a spot spectrum. This line is near the Mg b 4 line of 5167A.


Page 193. In 1869 Lockyer used a 6.2 inch (155mm) refractor F 15, giving a 0.9 inch (23mm) sun image, (see page 173) with a spectroscope, "the lines observed to be thickest in the spot spectrum."


Page 198-199. Oscillating slits by Lockyer to view prominences. Mechanically awkward. So he changed to an open slit method, which was tangent to the solar limb. Excellent prom detail.


Page 299. Hale, 60-foot tower, Mt Wilson, "most of the lines of the sun spot spectrum are merely widened by the magnetic field, but others are split into separate components, which can be cut off (with quarter wave plate and Polaroid) at will by the observer." Used a 600 gr/mm grating. Linear dispersion about 0.7A/mm in the second order.


Page 303-304. Hale with the 150-foot solar tower, Mt. Wilson, observed very slight widening of the photospheric lines in the polar region of the sun. Trying to measure the magnetic field, estimate about 50 gauss. Linear dispersion about 0.2A/mm (or 5A/mm).

His estimate was too high. Few decades later with better equipment, the polar field was determined to be about one gauss.


A Zeeman sensitive photospheric line away from a sun spot will be about 0.03A wide. The same line over a spot will widen to about 0.06A. Some lines are very sensitive and will widen to about 0.1A to 0.3A. Need about 3A/mm dispersion as a minimum.