April 14, 192 1] 



NATURE 



217 



telephony " was also discussed, but its practical use 

 in this country would be very limited. 



The sensitising of photographic emulsions for green 

 has always presented difficulties The well-known 

 "gap in the green" of orthochromatic plates, which 

 caused certain natural greens to be rendered too dark, 

 is perhaps the most notable of the irregularities. We 

 learn from a communication of Dr. Konig's in this 

 month's Colour Supplement of the British Journal of 

 Photography that Dr. Robert Schuloff, of the Hochst 

 dye works, has prepared a new dye, " pinaflavol," 

 which Dr. Eder finds to be " the long-required green 

 sensitiser, having a maximum at about the line E, 

 falling sharply to D, and extending without gaps to 

 F. ... It yields a strong, even, spectrum band over 

 the whole of the green, blue, and violet." The rapid 

 fall of sensitiveness at D is of especial advantage in 

 three-colour photography, as the green record can be 

 taken with a yellow filter which can easily be obtained 

 of great transparency to green. Hitherto it has been 

 necessary to cut off the red as well as the blue by 

 means of a green filter, and all green filters reduce 

 very notably the very colour that it is desired that 

 they should transmit. Pinaflavol is used in the same 

 manner as the cyanine and isocyanine sensitisers. 



The salving of the Italian battleship Leonardo da 

 Vinci forms the subject of an illustrated article in the 

 Engineer for March i8. This ship was blown up at 

 anchor at Taranto in 1916, the rent in the hull 

 measuring more than 500 sq. ft. and extending up 

 both sides. The vessel settled down by the stern, 

 capsized to port, and sank in six fathoms of water. 

 She is 650 ft. long, the displacement is 22,380 tons, 

 and she is armed with thirteen 12-in. guns. Being 

 extremely valuable, a committee was set up to report 



on different schemes of salvage and to arrange for 

 carrying out the work. It was finally decided to re- 

 float the ship upside down by meanS of compressed 

 air, to tow her into the Taranto dry dock, and there 

 to repair the damage so that she could afterwards be 

 righted at sea. The superstructure, turrets, guns, 

 etc., were detached and left provisionally at the bottom 

 of the sea in order to permit the vessel to enter the 

 dry dock in an inverted state. The whole of the pro- 

 jected work has now been accomplished, and the ship 

 was righted on January 24 last. The salvage of this 

 vessel constitutes a most remarkable and unpre- 

 cedented feat. It is also notable from the engineer- 

 ing point of view, since it has proved possibilities for 

 the use of compressed air which had not previouslv 

 been put to the test. 



A VERY useful catalogue (New Series, No. i) of 

 second-hand books and journals dealing with zoologv 

 has just been received from Messrs. Wheldon and 

 Wesley, Ltd., 38 Great Queen Street, W.C.2. It con- 

 tains the titles of no fewer than 2481 works (many 

 from the library of the late F. Du Cane Godman) in 

 the departments of Pisces, Reptilia and Batrachia, 

 Aves, Mammalia, Anthropology, Domestic Quadrupeds 

 and Birds, General Systems and Early Treatises, and 

 General Faunas; also text-books and miscellanea. 

 The catalogue can be obtained free of charge upon 

 application to the publishers. 



On p. 85 of our issue for March 17 we referred to 

 Mr. A. C. Kinsey's papers on American Cynipidae 

 or gall-wasps. Owing to an oversight they were 

 attributed to the Proceedings of the U.S. National 

 Museum, whereas they were published in Bulletin 42 

 of the American Museum of Natural Historv. 



Our Astronomical Column. 



Discovery of Pons-Winnecke's Comet. — The 

 comet Pons-Winnecke was detected by Prof. Barnard 

 on April lod. 2ih. 170m. G.M.T., R.A. i5h. 54m. 38s., 

 N. decl. 36° 38'. Daily motion 50' in north following 

 direction. The indicated date of perihelion is June 11 

 or 12. There will be a fairly close approach to the 

 earth, but no collision. Meteors are verv probable 

 about June 27. 



Reid's Comet.— This comet is brightening and 

 coming into a more convenient position for European 

 observers. Many observations are reported, the latest 

 being made at Copenhagen by Miss Vinter-Hansen : 

 G.M.T. April 4, i4h. 47-9m., apparent R.A. 

 2oh. 26m. 3I-40S., apparent S. declination 2° 38' 5*. 



On April 2 the comet was described as smafl and 

 bright, about 8th magnitude, with strong central con- 

 densation, no tail seen, but moon bright. It remained 

 visible in the dawn as long as 9th magnitude stars. 

 The orbit and ephemeris given in Nature for March 31 

 are not much in error, and there is everv reason to 

 anticipate that the comet will attain faint naked-eye 

 visibility. It will pass close to the North* Pole in mid- 

 May. 



Double Stars. — Mr. J. Jackson contributes an 

 article on this subject to, the Observatory for March, 

 NO. 2685, VOL. 107] 



in which he examines the criteria for distinguishing- 

 physical pairs from optical ones. It is pointed oiit 

 that two stars of the 9th magnitude, or brighter, 

 within 5" of each other are likely to form a phvsical 

 pair. Wide pairs with appreciable relative motion are 

 in most cases optical; without appreciable relative 

 motion their state is doubtful unless there is a con- 

 siderable common proper motion. 



If the relative motion of a pair of stars is less 

 than P.M./ 10, they are probably binarv. Some ob- 

 servers have been very reluctant to admit the first 

 pt-inciple, and have questioned the binarv character 

 even of such obvious pairs as 61 Cvgni. ' 

 ^ Mr. Jackson applies his principles to selected por- 

 tions of Burnham's General Catalogue, classifvin^^ 

 several stars as obviouslv binary, others as almos't 

 certainly optical. He passes on to consider the hvpo- 

 thetical parallax, on the assumption that the mass of 

 each system is twice that of the sun. He shows that 

 this may often be estimated, even if onlv a small por- 

 tion of the orbit has been observed; where its value 

 IS large the star should be put on the list of parallax 

 stars. If the observed parallax is not verv different 

 from^ the hypothetical one, the star is probablv a 

 physical binary Thus /? 4972 and /3 7514 are shown 

 to be respectively physical and optical. 



