34 PROCEEDINGS OP THE AMERICAN ACADEMY 



multiplying by 0.446 and dividing by 1.446 will give a motion of 

 Algol of 0".027 in each revolution. This corresponds to 3".43 an- 

 nually. If the parallax of Algol is O'M, this would correspond to a 

 velocity of about 160 kilometers (100 miles) per second. Substituting 

 the values in the equation on page 9, t; = 13 /, we have 1= 2.6, or 

 the F line would be deviated through an interval equal to nearly half 

 the space between the D lines. Moreover, as this quantity would be 

 alternately positive and negative every thirty-five hours, the system- 

 atic errors which are so troublesome in such measures could be 

 eliminated, and the quantity to be observed would be doubled. If 

 the parallax of the star is more than 0".l the motion would be less, 

 but on the other hand the parallax would then become a suitable 

 object for micrometric measurement. If the parallax is much less 

 than O'M the motion would be so large that its variations might be 

 determined with some accuracy, and the form of the orbit computed 

 from the varying velocity along the line of sight. These measures 

 would also determine the dimensions of the orbit, and if we assume 

 the value of the briglitness, I, they would give the distance and par- 

 allax of the star. The spectrum of Algol has already been examined 

 without the detection in it of any peculiarity. The time selected for 

 observation would be more likely to be near its minimum, to detect 

 any changes in the spectrum accompanying its variation in light. But 

 this is the very time when the motion along the line of sight is zero, 

 which may be the reason why this phenomenon has as yet escaped 

 detection. 



Two objections have been offered to the theory that tlie variation 

 in light was due to the interposition of a non-luminous satellite. First, 

 the large size of the satellite ; and, secondly, the rapidity of its motion. 

 It has been said that, according to the prevalent theories regarding the 

 formation of the stars, so large a body could not well have lost all its 

 heat while the lumhious star is still so bright. This argument would 

 have some force, if we were sure of the true origin of the stars, and 

 also if we knew that both bodies are of the same age. They may, 

 however, have had a wholly independent origin, and have come together 

 through their proper motion, under the influence of a resisting medium 

 or other disturbing force. 



The objection to the rapidity of motion cannot be defended in tliat 

 form. By tlie law of gravitation we can compute what should be the 

 velocity with a given density, and the only proper criticism would be 

 that to produce the observed velocity an improbable density would 

 be required. To determine this density we may use the formula of 



