Physical Constitution of the Sun. 301 



value 10 times as great, the rest of the assumptions remaining 

 unaltered, would only lower the calculated temperature to 



*= 47270°. 



Both from this circumstance, and from the simplicity of the 

 theoretical hypotheses, the formula here given may claim an 

 essential superiority over the way I formerly proposed for the 

 determination of the temperature of the sun. Besides, it exhi- 

 bits a remarkable relation between the quantities t and h. If, 

 namely, the altitude of the chromosphere were really determined 

 only by the ratio of the densities at the lower and the upper 

 boundary, the distance h of the two strata in which this constant 

 pressure- or density-ratio exists would change proportionally to 

 the absolute temperature. Hence, if we had numerous obser- 

 vations, to be treated statistically, on the mean altitude of the 

 chromosphere at all parts of the solar margin, we should hereby, 

 without knowing the temperatures themselves, be able to ap- 

 proximately judge of their ratio at different parts of the sun's 

 surface — for instance, at the equator and the poles. 



The small altitude of the chromosphere at the poles of the sun 

 found by Secchi to correspond with his observations seems, in 

 conjunction with the other results of the same observer, on the 

 slight heat-radiation of the polar regions of the sun, to confirm 

 the above-deduced relations*. For instance, at parts of the sun's 

 surface where the mean height of the chromosphere amounts to 

 about 15", on the above-made hypotheses an absolute tempera- 

 ture of about 90,000° would result. But, however important 

 for investigations of the temperature-proportions on the solar 

 surface the proportionality between the temperature and the alti- 

 tude of the chromosphere may become in the future, for obvious 

 reasons the significance of this connexion need not be overesti- 

 mated beforehand. 



In order now to illustrate the applicability of formula (11) 

 by the insertion of numerical values, let it in the first place be 

 remembered that the comparability of the phenomena observed 

 in Geissler's tubes with those in the chromosphere is only ad- 

 missible on the hypothesis that the essential conditions on which 

 the phenomena compared depend agree in the two cases within 

 certain limits. As such conditions two have been substantially 

 recognized — namely, first, the temperature, and, secondly, the 

 quantity of the gas-mass which is encountered by the observer's 

 visual line in equal cross sections of the spaces passed through. 



* The depression of the chromosphere over the sun-spots, observed by 

 Respighi might likewise be ascribed to a lowering of the temperature at 

 those places. 



