October 20, 1916] 



SCIENCE 



573 



them in the order of the angular separation 

 of the two components. We thus get the fol- 

 lowing means, the number in parentheses indi- 

 cating how many pairs are included in each 

 mean: 



MEASURED SEPARATIONS 

 With the 12-Inch With the lSK-mch 



0.94 

 1.80 

 2.34 

 3.29 

 4.18 



1.14 

 1.96 

 2.43 

 3.32 

 4.25 



With the 12-Inch 



0.68 



1.13 

 1.71 



2.05 

 2.55 

 3.96 

 4.65 



With the ISH-Inch 



(5) 



(5) 

 (5) 

 (5) 

 (5) 



With the 40-Inch 



o'.'85 (5) 



1.34 (6) 



1.87 (4) 



2.22 (6) 



2.57 (5) 



4.03 (8) 



4.66 (6) 



With the 40-Inch 



0.83 

 1.54 

 1.98 

 2.45 

 3.54 

 4.34 



0.88 

 1.59 

 1.93 

 2.50 

 3.55 

 4.29 



(5) 

 (6) 

 (6) 

 (6) 

 (6) 

 (7) 



Measures made with the two large telescopes 

 show little or no systematic difference, but 

 those made with the 12-inch yield smaller 

 separations than either of the others, the differ- 

 ence being largest for small separations and 

 becoming negligibly small for separations in 

 the neighborhood of 5". 



In the recently issued Volume 12 of the 

 Publications of the Lick Observatory, Pro- 

 fessor Aitken gives a long list of measures of 

 double stars. Many of these were examined 

 with both the 12-inch and the 36-inch tele- 

 scopes of that observatory, so that we have an 

 opportunity for making the same kind of tests 

 as on Professor Fox's observations. The re- 

 sults similarly collected are as follows : 



MEASURED SEPARATIONS 



With the 12-Inch With the 36-Inch 



0'.52 0'.42 (20) 



0.62 0.54 (25) 



0.71 0.64 (20) 



0.81 -. 0.79 (24) 



1.07 1.03 (24) 



1.38 1.39 (21) 



2.13 2.10 (26) 



4.49 4.53 (18) 



Here again we have a systematic difference 

 that increases as the separation becomes 

 smaller. But in Professor Aitken's measures 

 the difference has the opposite sign from Pro- 

 fessor Pox's, the measures with the smaller 

 telescope coming out larger than with the 

 greater telescope. It would be interesting to 

 know whether these are instrumental peculiar- 

 ities or whether they have their origin in the 

 habits of the observers. In any case it seems 

 likely that a discussion of the systematic 

 errors of telescopes and observers would be well 

 repaid in the additional accuracy with which 

 double-star orbits could be computed after the 

 application of systematic corrections. Some 

 attempts at such a study have been made, but 

 (so far as the reviewer is aware) none of them 

 is as thorough as the importance of this mat- 

 ter warrants. Needless to say that the pres- 

 ence of systematic errors of this kind is evi- 

 dence for the skill and the care of the observer 

 rather than against. In the work of an in- 

 experienced or careless observer, such small 

 effects as these would be buried under an 

 accumulation of accidental errors. 



Prank Schlesingee 

 Allegheny Observatory of the 

 University of Pittsburgh, 

 September 30, 1916 



The Sessile Barnacles (Oirripedia) contained 

 in the Collections of the U. S. National 

 Museum; including a Monograph of the 

 American Species. By H. A. Pilsbry. 

 Bulletin 93, U. S. National Museum, 1916. 

 In this great work, of 366 pages with 76 

 plates, Dr. Pilsbry brings the American ses- 

 sile barnacles out of obscurity, and furnishes 

 the means whereby all who will may continue 

 the investigation of the group with as much 

 ease as the nature of the subject permits. A 

 critical review of the book could only be 

 written by one who had covered at least a con- 

 siderable part of the field by his investigations, 

 and at present Dr. Pilsbry stands alone in this 

 country in his knowledge of barnacles, with no 



