March 7, 1919] 



SCIENCE 



243 



tube on the spectro^aph body. The Xernst 

 filament was placed vertically. It was used at 

 such a distance from the slit that at its nearest 

 position to the slit the whole of it was effective 

 in illuminating the plate. 



The graduations of the sector opening were 

 compared and corrected with a protractor. The 

 sector wheel ran 120 revolutions per minute, 

 there being two openings in the wheel, the sum 

 of the two being an angular opening equal to 

 that given in the table. 



The plates used were half of them Wratten 

 & Wainwright Panchromatic and half Seeds 

 Panchromatic. In one instance a Seeds Proc- 

 ess was used with the same results. The 

 plates were given uniform tank development. 

 The exposures were such as to give rather 

 faint images, necessary in order to judge ac- 

 curately differences in intensity. The ex- 

 posures were such, however, that with the sec- 

 tor running they were always longer than one 

 jninute. A plate would contain a set of ex- 

 posures for the so-called zero of the experiment, 

 the initial balance distance, and a set of ex- 

 posures with the sector running, the movable 

 lamp being placed at distances such as to make 

 equivalent sets of exposures. The distances 

 corresponding to the two pairs which matched 

 on such a plate when divided the one by the 

 .other gives a quotient which is a figure of the 

 table. With a good setting the two spectral 

 bands balanced throughout if they balanced at 

 all, showing that the proposition is independent 

 of wave length. 



As the sector photometer is used for spectro- 

 photometry the two beams fall on a bi-prism in 

 front of the slit with the result that the two 

 beams on the plate are in juxtaposition. Be- 

 cause of the fact that the total reflecting prism 

 used here had been slightly ground on its edges 

 the two bands of the pair in this experiment 

 were .4 mm. apart, which increased somewhat 

 the difficulty of judging equality in blackening. 

 The error in such judgments was probably of 

 the order of 2 per cent. It may have been less 

 than this. The averages for the figures of the 

 table differ as the last two columns show by 

 About a half per cent, from what the figure 

 should be if the diminution in the intensitv 



of the beam due to the sector is photograph- 

 ically equivalent to the diminution due to a 

 proportionate increase in distance. 



That this equality exists is certainly a co- 

 incidence. Recently Helmick^ has shown that 

 long exposures produce less blackening than 

 short e.xposures, the total energy being the 

 same (this being when both the short and long 

 exposure are longer than a certain fixed time). 

 In some rough experiments which I first made 

 I found that the total actual intermittent ex- 

 posure necessary to produce equal blackening 

 through a 72° sector was about of the order of 

 12 per cent, longer than for a like continuous 

 exposure, i. e., the sector at rest. The evidence 

 herein contained goes to show that when the 

 beam is dimmed by increasing the distance of 

 its source the exposure must likewise be longer 

 by this same amount. In other words, if 5,, 

 £^ and B^ are the blackenings due respectively 

 to a certain beam, to the same beam made in- 

 termittent and to a beam of decreased inten- 

 sity, all of the beams delivering equal total 

 energy through the regulation of the time fac- 

 tor, then B, and B^ are less than B^ but are 

 equal to each other. 



H. S. Neavcomer 



The Laboratosy or the 

 Henry Phipps Institute op the 

 University op Pennsylvania 



THE AMERICAN ASSOCIATION OF 

 VARIABLE STAR OBSERVERS 



The seventh annual meeting of the American 

 Association of Variable Star Observers was held 

 at the Harvard Observatory, Cambridge, Mass., 

 on November 23, 1918. More than a score of the 

 members were present and the association became 

 formally incorporated under the laws of Massa- 

 chusetts. The meeting was, without doubt, the 

 most successful and enjoyable that has yet been 

 held. The reports of the several committees indi- 

 cated the active interest and aims of the mem- 

 bers, and a new committee, under the chairman- 

 ship of Professor S. I. Bailey, was appointed to 

 gather together a collection of astronomical slides 

 which could be loaned, under proper supervision, 

 to members who might care to lecture in their 

 vicinity, thus tending to arouse a greater interest 

 in astronomy and particularly variable stars. 



2 P. S. Hebnick, Phys. Eev., XI., 5, 1918, 372. 



