282 PROCEEDINGS OF THE AMERICAN ACADEMY 



transit instrument, or small equatorial. The first of the three methods 

 of estimating their light is to be recommended for this work. It is 

 sufficiently precise, and the scale used each evening would be readily 

 found fi'om the Durchmusterung magnitudes of the great mass of the 

 stars which would, probably, be invariable in light. Any interesting 

 variable would be detected by observations on a few nights. 



5. Measures of the position angles, distances, and magnitudes of the 

 companions. The approximate places given from the Durchmusterung 

 in Table XIII. could tlius be corrected, and the blanks for southern 

 stars filled. The magnitudes could best be measured by the photome- 

 ter recommended in (2). Otherwise especial care should be taken 

 that the light of the fainter star was not affected by the proximity of 

 the brighter. 



6. Observations of the color and spectrum of these stars, to decide 

 which ones, if any, should be included in tlie second class. 



7. Distribution of the light in the spectra of these stars, and also 

 of those of the second class at their maxima and minima. 



8. Computation by Jacobi's method of the true diameter of a liquid 

 ellipsoid in equilibrium, having given the period of rotation and the 

 ellipticity of the equator. 



9.. Computation of the Galactic latitude and longitude (or distance 

 and direction from the pole of the Milky Way) of variables of Classes 

 II. and IV., of the planetary and other gaseous nebulae, and of stars 

 whose spectrum is of the fourth type. 



10. Computation of the position of the poles of the orbits of the 

 binary stars. 



The object of the present paper is not to advocate a certain theory 

 which may seem improbable, and, possibly to some, inadequate. It is 

 rather intended to bring together the most important facts bearing on 

 the study of an interesting class of objects, and to exhibit them in a 

 form in which they may be subjected to any desired test. The hy- 

 pothesis advanced has a value as affording a simple geometrical con- 

 ception of the nature of the variations under consideration, even if it 

 proves not to be the true explanation of the cause. The ingenious 

 hypothesis of Zollner, and other explanations of these phenomena, have 

 not been overlooked. It seemed best, however, to leave to another to 

 decide the comparative merits of views in which the precision of the 

 effects must be considered as well as the probability of the causes. 



One theory, that the variation is due to the absorption of a rotatmg 

 mass of gas, deserves a moment's consideration. This explanation 

 does not appear probable for stars of the fourth class, since no evidence 



