ASTEONOMICAL PHENOMENA AND PROGRESS. 



43 



0.280* aperture. After many trials, he suc- 

 ceeded in obtaining, by short exposures, a well- 

 defined, nearly circular photographic image 

 of stars of the first and second magnitudes. 

 AVitli exposures of eight minutes, stars as low 

 as the ninth magnitude, when white, give a 

 photographic impression. He says that the 

 greatest difficulty in stellar photography is to 

 make, the image stationary on the plate during 

 a long exposure. The steadiness is absolutely 

 necessary for the production of circular images; 

 the images must be circular, because in ellipti- 

 cally lengthened images the eye cannot fix the 

 centre with the sharpness required for the 

 measurements. Employing even the most per- 

 fect clock-work, the steadiness of the image is 

 affected by the effect of other atmospheric 

 representations, by the variations in the re- 

 fractions caused by atmospheric disturbances, 

 and by the increase of refraction dependent 

 on the zenith - distance. The photographic 

 image of stars is circularly spread by prolon- 

 gation of exposure ; this is principally the effect 

 of the scintillating motion of the image, not 

 of want of definition, as its amount depends 

 on the state of the atmosphere. "When the 

 state of the atmosphere produces a strong 

 scintillating motion, the images of bright stars 

 become very large by long exposures, and 

 faint stars do not produce any impression. 

 In great zenith-distances, another obstacle, the 

 strong absorption of chemical rays by the at- 

 mosphere, combines with the increased scin- 

 tillation. Prof. Sellack found that the favor- 

 able time for taking photographs is, for north 

 culminating stars, near culmination ; for stars 

 culminating near the zenith, at some distance 

 from culmination; nnd for south culminating 

 stars, near culmination. He has photographed 

 some twenty star-clusters, most of them in the 

 constellation Argo, and some in Canis Major 

 and Scorpio. The cluster near Carintc Argus 

 gave 123 stars on the photograph in the most 

 favorable night. Other groups yielded from 

 forty to sixty stare; and the total product of 

 twenty-seven clusters was 800 stars. This is 

 not disappointing. The Pleiades, the richest 

 Northern group, gives to Mr. Kutherfurd's 

 photograph only forty-five stars. 



Srintillntion of the Start. In a note to 

 Nature Mr. George F. Burder records an inter- 

 esting observation on this subject. He re- 

 marks that when a person selects two stars 

 of suitable size and distance apart, and looks 

 ilastly nt one, the other being at the same 

 time within the range of vision, the star which 

 is not looked at will become alternately visi- 

 ble and invisible. On a favorable evening, 

 and the scintillation of the stars being very 

 marked, be made this experiment on the stars 

 Kpsilon and Zeta of Ursa Major. Looking 

 steadily at one, he noticed that the other be- 

 came every now and then totally invisible, for 

 a period of some duration. On one occasion 

 h>- counted thirty in the interval of disappear- 

 ance, and this he afterward found to corre- 



spond to five seconds. More frequently the 

 star would be invisible for one or two sec- 

 onds, then flash into full brilliancy, and, after 

 a variable interval, vanish as suddenly again. 

 He also tried two of the bright stars in Orion, 

 and in that case the apparent extinction of 

 one of them, from time to time, produced an 

 effect almost startling. The author does not 

 suggest any explanation of this. It is, in fact, 

 a common optical phenomenon, which may be 

 seen by trial upon two gas-lamps at the proper 

 distance apart. 



Mapping the Southern Heatent. Dr. B. A. 

 Gould, director of the Cordova Observatory, 

 in a letter to the American Journal of Science, 

 dated August 6, 1873, records the continued 

 success of his labors there: Notwithstanding 

 the unfavorable weather with which he had 

 to contend during the greater part of the 

 time, he had mapped 879 zones, containing 

 about 50,000 stars, of which less than one-fifth 

 are duplicates. From present means of judg- 

 ing, he expects to find about 65,000 stars in 

 the 500 zones lying between the parallel of 

 23 and 80 southern declination. The south- 

 ern circu in polar list of six stars which Dr. 

 Gould proposed for the Coast Survey about 

 fifteen years ago, affords him now an ample 

 series of stars for determining the azimuth at 

 any desired time, and obviates the neeesMty 

 of too long an interval for the observation of 

 determining stars between the zones. The 

 work upon which Dr. Gould is now especially 

 engaged will require for its completion the 

 greater part of another year. 



Recent Entimatet of Solar Temperature. 

 Mr. James Dewar, in a paper read before the 

 Royal Society of Edinburgh, groups all the 

 known methods of arriving at a knowledge of 

 high temperatures, under eight different pro- 

 cesses. He then goes on to show how it is 

 possible, from the luminous intensity of the 

 sun, to devise a new estimate of solar temper- 

 ature. This calculation is based on a definite 

 law, viz., that the total luminous intensity is 8 

 parabolic function of the temperature, above 

 that temperature where all kinds of Inminons 

 rays occur. The temperature of the oxyhy- 

 drogen-flame does not exceed 2,400 C., nnd 

 sunlight, according to Fiscau and Foncalt's 

 experiments, has 150 times the luminous in- 

 tensity of the lime-light; so that it is only 

 necessary to calculate at what temperature this 

 intensity is reached, in 'order to get the solar 

 temperature. This temperature is 16,000 C. 

 in round numbers. Enormously high tempera- 

 tures are not required to produce great lumi- 

 nous intensities, nnd the temperature of the 

 sun does not nt least exceed the above esti- 

 mate. Mr. Dewar cites the opinion of Sir 

 Wm. Thomson, who says: "It is almost cer- 

 tain that the sun's mean temperature is even 

 now as high as 14,000 C.," an estimate which 

 agrees well with the luminous intensity calcu- 

 lated. 



Spectrum of the Sun. In a paper contrib- 



