6t4 SCIENCE PROGRESS 



August 1 6, when its magnitude, on a photograph of the field 

 in which it appeared, taken in Sweden, was reckoned as the 

 seventh. When discovered by Mr. Denning on August 20 it 

 had risen to magnitude 3" 7, and it attained its maximum, r8 

 magnitude, on August 24. The difference between the magni- 

 tudes 16 and 2 would indicate an increase of luminosity about 

 four hundred thousand fold. 



On the other hand, a new star may be one which was known 

 as existing previously, but which suddenly received an extra- 

 ordinary access of luminous energy. Such was Nova Aquilae 

 (191 8). It had been recorded as a star of the eleventh magni- 

 tude on the Harvard College photographs in 1888, and again 

 at Algiers in 1909. Its light was known to be slightly variable 

 to the extent of about half a magnitude. On June 3, 191 8, it 

 was a star of the eleventh magnitude. On June 7 it had risen 

 to the sixth magnitude, a hundredfold increase in light. 

 Discovered on June 8, it rivalled Sirius in brilliancy in the 

 course of the next twenty-four hours, so that in less than six 

 days its luminosity had increased more than twenty-five thou- 

 sand fold. 



The rate of discovery of new stars during the first eighteen 

 centuries of the Christian era was on the average one for each 

 century. Of these the most famous was that which appeared 

 in the constellation Cassiopeia, in November 1572, and which 

 was assiduously observed by the famous Danish astronomer, 

 Tycho Brahe. He was then twenty-six years of age, and he 

 has left to posterity a full account of his observations in his 

 work entitled De Nova Stella. When the opening year of the 

 twentieth century was marked by the appearance of Nova 

 Persei, which year, too, coincided with the tercentenary of the 

 death of the distinguished astronomer, the Royal Society of 

 Copenhagen issued in photogravure a reproduction of Tycho 's 

 book, of which at that time only three copies were extant. 



Of this celebrated nova we learn that it rivalled the planet 

 Venus in brilliancy, and indeed was visible in broad daylight 

 at noontide. It was not seen after the month of March 1574, 

 but in the meantime its colour had changed from white to red, 

 and then again to white, indicative, no doubt, of its changing 

 spectrum, on which we shall dwell more at length in what 

 follows. Besides discussing the colour changes in the star, 

 and its varying magnitude, Tycho also, from his measures, 

 came to the conclusion that the appearance was not one that 

 belonged to our atmosphere, but appertained to the sphere of 

 the fixed stars, and consequently was very distant. The book 

 is replete, too, with astrological lore and prophecies. The 

 appearance of the star is likened, as a wonderful portent, to 

 the standing still of the sun at the command of Joshua, and 



