ASTRONOMY. 1 73 



0.25 X secant of the zenith distance, i. e., a star seen from the outside 

 of our atmosphere would appear a quarter of a magnitude brighter 

 than it does now in the zenith, and half a magnitude brighter than at 

 00° from the zenith. The magnitudes given in the final catalogue are 

 all reduced to what they would appear if the star were seen in the 

 zenith. It would be interesting to see some careful determination of 

 the atmospheric absorption made within the last year, for, in the opin- 

 ion of many observers, the transparency of the atmosphere and the 

 brightness of all the stars have sensibly diminished ever since the red 

 sunset phenomena following the Krakatoa explosion in August, 1883, 

 and by not a few all these effects, and the still continuing dirty pinkish 

 red corona around the sun, are attributed to Krakatoa dust still over- 

 head, though now at a considerably lower level than at tirst. 



In all this work the constancy of brightness ►of the pole star was of 

 course a much-to-bedesired condition, and this question received very 

 careful investigation, and there seems to be no doubt of its reality so 

 far as photometric measurements can determine. Besides the use of 

 the long series of observations upon the 100 circumpolars, a special 

 series of naked-eye comparisons of Polaris, with several neighboring 

 stars of nearly equal brightness, was undertaken for this purpose, and 

 in connection with the latter some very interesting results come out, 

 showing that there is in the case of some observers a very decided dif- 

 ference (and very likely to some extent with all) in the apparent relative 

 brightness of two stars dej^ending upon their relative positions right 

 and left and up and down. This had been shown in the photometric 

 work, so that in each comparison the two images in the field of the 

 photometer were reversed right and left between the two pairs of set- 

 tings of the Nicol. It is interesting to tind that it also aflects naked- 

 eye estimates, in spite of the fact that in the latter the endeavor is to 

 look steadily at one star and then at the other alternately. 



The most imiiortant question of all is the scale of magnitudes adopted. 

 Professor Pickering has adopted that proposed some years ago by Pog- 

 son, the director of the Madras Observatory. 



On this scale, then, we receive from a sixth-magnitude star, which is 

 about the limit of naked-eye vision, just one-hundredth part as much 

 light as from a first-magnitude (about one-fortieth that of a second, one- 

 sixteenth that of a third, three-nineteenths that of a fourth, and two- 

 fifths that of a fifth-magnitude star). This is the first time that the 

 magnitudes of the stars have been measui'ed and catalogued on any 

 such extensive plan or with any such degree of accuracy, and at the 

 same time upon a true geometrical light-ratio scale, and it marks an 

 epoch and an entirely new starting i)oint in stellar photometry. Hith- 

 erto there has been much confnsion in this matter, although a j^retty 

 general adoption of Argelauder's scales of magnitudes has been slowly 

 mending this. But the work of Professor Pickering must inevitably, 

 froni its intrinsic merit, at once replace all existing scales of mngni- 



