OF ARTS AND SCIENCES. 19 



plain the variations of stars of the fourth class. Another theory would 

 account for the changes of liglit by an opaque body or satellite passing 

 between the star and the observer. It will be the object of the fol- 

 lowing discussion to show how fully this explanation will account for 

 the variations of stars of the fifth class. A modification of this theory 

 would replace the single eclipsing body by a cloud of meteorites. Su(;h 

 a theory will account for almost anything by suitably modifying the dis- 

 ti-ibution of the meteors. If we can show that all the effects may be 

 explained by a single body, or what amounts to the same thing, a 

 spherical cloud of meteors so dense as to be opaque, there seems to be 

 no reason for assuming a cloud of another form. All that can be 

 claimed for any theory is that it explains all the facts. If then the 

 computed variations of light agree with the observations within the 

 limits of errors of observation, that is all that can be asked, and the 

 theory should be accepted as the most probable explanation until some 

 new fact is discovered which it will not explain, or some new theory 

 which agrees equally well with observation and appears to be less 

 improbable. The diminution in light might be caused by the inter- 

 position of a body which was self-luminous, instead of dark. We 

 should then have a close double-star, one component of which passed 

 in front of the other. If the orbit was circular, we should have two 

 min-ma during each revolution, and at these times the star would 

 appear of unequal brightness, unless the intrinsic brightness of the 

 two bodies was the same. When the darker body passed in front of 

 the brighter, the light would be less than when the brighter passed in 

 front of the darker. If the orbit was elliptical there might be only 

 one minimum. In the case of Algol more than half the light is cut 

 off at the minimum; consequently one body must be darker than the 

 other. As no second minimum has ever been observed, it is probable 

 that the eclipsing body is not self-luminous. 



We must now show that neither of the other theories named above 

 will explain the variations of Algol and of other stars of the fifth class. 

 The regularity of the variation disposes of the theory of a volcanic 

 eruption, a collision, or a system of sun-spots. These effects also could 

 scarcely be repeated so frequently without exhausting the source of en- 

 ergy from which they were derived. The theory that the variation is 

 due to the revolution of the star appears more probable, and the regular- 

 ity and shortness of the period add weight to it. On the other hand, 

 it is difficult to account by this theory for the sudden changes in the 

 light. If the light was reduced by a dark portion of the star being 

 turned towards the observer, the minimum should last until, by the 



