170 



KNOWLEDGE 



[September 1, 1892. 



through which we look up at all the phenomena of the 

 outer universe. 



The bright light from the body of the sun lights up the 

 dust in the atmosphere, and forms a luminous veil which is 

 drawn over the whole sky and hides from us, under all 

 ordinary circumstances, the brilliant prominences and fainter 

 corona which surrounds the sun. At first it was only when 

 the dust veil was rendered transparent by being plunged into 

 the shadow of the moon that the veil was, as it were, 

 rent for us, and for a few moments, during a total eclipse, 

 we were permitted to look upon the wonderful structures 

 about the sun. Then came the method of stretching out 

 into a spectrum the white light dispersed by the dust veil, 

 and observing upon the band of colour the unstretched 

 coloured images of the gaseous structures behind the white 

 veil. But, under all ordinary conditions, when the coloured 

 images are bright enough to photograph, only a narrow 

 slit can be looked through, showing a very narrow slice of 

 the solar structiu'e. 



The ingenious advance made by Prof. Hale of Chicago, 

 which has enabled him to photograph large prominences 

 and monochromatic images of the whole disc of the sun, 

 consists in employing a moving slit which is carried across 

 the prominence intended to be photographed, or across 

 the whole image of the sun, by a clepsydra, while another 

 slit is evenly moved at a corresponding rate just in front of 

 a photographic plate, so that the slit only allows one 

 coloured image to fall upon the sensitive plate and screens 

 it from the action of all other coloured images, and a 

 photograph printed by monochromatic light is obtained. 



The apparatus at first used consisted of a cylinder in a 

 closed box, at the eye end of the observing telescope of a 

 large diffraction spectroscope. The axis of the cj'liuder 

 was parallel to the lines in the spectrinn, and the cylmder 

 could be rotated at a uniform rate by a small clepsydra 

 driven by a supply of water mixed witli spirits of wine, so 

 as to avoid the chance of freezing m cold weather. A strip 

 of flexible celluloid photographic film on the circumference 

 of the cylinder was slowly moved in the plane of dispersion 

 behind a narrow slit at the focus of the observing telescope. 

 The diffraction grating was rotated until the k line in the 

 foiu-th order spectrum passed through the slit and fell 

 upon the sensitive film. By changing the rate of the 

 driving clock of the telescope, the sun's image was made to 

 drift slowly across the first slit of the spectroscope, while 

 the film rotated at the proper speed. In this way, if the 

 motion of the cylinder is properly proportioned to the 

 motion of the driving clock, a chcular image of the sun 

 can be built up. In the image of the sun shown on the 

 lower half of our plate the two motions were not 

 properly proportioned, and consequently the image is 

 elliptical. Our plate is by no means a satisfactory one. 

 The glass photograph kindly sent by Prof. Hale as a speci- 

 men of his work, shows two bands of bright facuhe running 

 across the sun's disc in the region where spots are most 

 frequently found. 



The facul*, shown in this photograph, all correspond 

 to the bright monochromatic solar image k, though no 

 doubt they shine with other light as well, but the k line is 

 generally the most active photographically in the spectra 

 of the prominences, to which the faculse are evidently 

 allied. Under ordinary circumstances, when examining 

 the Sim with a telescope. facuLe of much less extent are 

 seen, and they are only clearly recognizable near to the 

 sun's limb. The photographs made by the new method 

 reveal the fact that they do not merely equal the spots in 

 area but that they are far more extensive — in fact the spots 

 sink into relative insignificance beside the facula? which 

 surround them. 



Prof. Hale has recently succeeded in making photographs 

 in which the faculff, spots, chromosphere, and prominences 

 are all shown on a single plate in their proper relative posi- 

 tions. This cannot be done with Prof. Hale's instrument 

 in a single exposure, as the time required to bring out the 

 prominences is much too long for the faculse. A diaphragm 

 covering the sim's image at the focus of the equatorial is 

 therefore employed, and the slits are made to move across 

 at the speed required for the prominences. At the end of 

 the stroke the diaphragm is removed, and the slits are 

 made to move back over the image at a much higher speed 

 by adjusting the valves of the clepsydra. An image of the 

 sun's surface is thus formed on the plate exactly within the 

 image of the chromosphere formed during the whole 

 exposure, and the whole operation can be completed in a 

 minute. One great advantage of the photographic method 

 is that sudden and short-lived eruptions, with which all 

 solar observers are famiUar, can be studied and measured 

 at leisure. 



The upper pictiu-es on our plate represent a fine 

 prominence photographed by Prof. Hale in May last. 

 Much of the faint and delicate detail visible in the glass 

 picture sent by Prof. Hale is lost in our copy, but our 

 readers will no doubt be able to recognize several dark tree- 

 like forms cutting out the light of the bright prominence. 

 These, no doubt, correspond to a cooler group of 

 prominences situated between us and the larger bright 

 prominence, and they are seen reversed on the bright 

 background. 



'^'e heartily congratulate Prof. Hale, and wish him stUl 

 further success in the fruitful hne of research which he 

 has made for himself. 



3ltttrrs. 



[The Editor does not hold himself responsible for the opinions or 

 statements of correspondents.] 



THE XOVA AURIG,E. 



j Extract from a letter from the Rev. T. E. Espin, (/nfcrf 

 Toulaif, Ddrihiyton, 22h(/ Aniiufyt, 1892. 

 . . . I had a letter from Mr. Henry Corder yesterday 

 telling me that the Nova was again visible. I looked it 

 up last night, and found it 9-2 mag., white or yellowish 

 white. The spectrum has changed to a monochromatic 

 one, all the star's hght being concentrated in one intensely 



i bright line, which is probably X oOO'O. The Nova, like 

 the Cvgnus one, seems to have become a planetary 

 nebula. . . .— T. E. Espix. 



[The changes observed in the spectrum of the Nova 

 Cygni as it vanished seem to show that the nebular type of 

 spectrum was not the last that it presented. Dr. C'opeland 

 and Lord Crawford at Dimecht noted the single greenish 

 line of the nebular spectrum, A 500 ±, as the only line in 

 the spectrum of the Nova on 2nd September, 1877''-\ 

 Some three years afterwards its spectrum was examined 

 with the ly-inch refractor at Harvard College Observatory, 

 and found to correspond with the spectrum of an ordinary 

 star. Dr. C'opeland was, in 1881, imable to see any trace 



: of light with the spectroscope at Duuecht, which is suffi- 

 cient proof that its spectrum could not then have been 

 monochromatic. By 1885 it had decreased to a star of 

 the 15th magnitude. Dr. J. G. Lohse, who examined it 

 with Mr. Wigglesworth's 15|-inch refractor near Scar- 

 borough, was unable to detect any • spectrum when 

 examined with the spectroscope. It then appeared bluish in 

 colour and nebulous. Thus the monochromatic nebular 



* Copernicus. Vol. II., p. lOS. 



