October 3, 1919] 



SCIENCE 



331 



out the Nortli American ranges of the species. 

 The casual records are added separately, but, 

 ■we regret to see, with altogether too little spe- 

 cific data. The egg dates are generalized rec- 

 ords taken from a great mass of data and are 

 usually given for one or two states with only 

 inclusive dates. 



Disclaiming any attempt at critical treat- 

 ment of the questions of relationship, our 

 author, however, occassionally adds comments 

 of this character. One of the most interest- 

 ing of these relates to that peculiar form Uria 

 ringvia,, which some ornithologists consider 

 a distinct species, and others a mere aberra- 

 tion of Uria troille. Mr. Bent presents the 

 data on both sides of this question, but seems 

 to think that the bird is a distinct species. 

 Other important critical remarks are given 

 under Gavia arctica arctica, which is shown 

 to be extralimital so far as North America is 

 concerned. The records ascribed to this are 

 considered all properly referable to the re- 

 cently described Gavia viridigularis Dwight, 

 which is here treated as a -subspecies of the 

 European Gavia arctica. 



" The Life Histories of North American 

 Diving Birds " is unusually well illustrated. 

 The 43 black and white full-page plates repre- 

 sent nearly twice that many scenes in the life 

 of the various species, and consist of half tones 

 showing habitat, nests, eggs, young, and some- 

 times also adult birds; many of these are of 

 much scientific interest and add greatly to the 

 instructiveness and interest of the book. The 

 12 colored plates represent the eggs of many 

 of the species. These are apparently of nat- 

 ural size, but there is, unfortunately, no indi- 

 cation on the plates or elsewhere that this is 

 the case. 



It is manifestly impossible in the brief space 

 of a review to do justice to this work, crowded 

 ■as its pages are with information; but one 

 thing we may say, and with truth, that " The 

 Life Histories of North American Diving 

 Birds " is one of the most important contri- 

 butions to North American ornithology, and 

 will for a long time be the recognized author- 

 ity on biography of the species that it treats. 



Harry C. Obeeholser 



SPECIAL ARTICLES 

 VISIBILITY OF BRIGHT LINES 



There has been a material amount of in- 

 vestigation regarding the visibility of dark 

 lines against a light background. Seeing a 

 linear object is much easier than seeing a spot 

 of similar minimum dimension, and totally 

 different from resolving parallel lines, which 

 ■must be distinct as a whole before there is the 

 ■least chance of resolution. In general terms 

 distinct lines or spots can, with difSeulty, be 

 resolved when distant 1', to judge from the 

 ■average of many experiments,^ depending on 

 relative contrast of the objects and other ex- 

 perimental conditions, and barring occasional 

 cases of highly abnormal acuity, V = 5-8, such 

 as those reported by Cohn.^ A single spot, 

 ■white on black or black on white can be de- 

 tected by one with fairly keen vision down to 

 ■a diameter of 30", by an occasional observer 

 to half this value, again depending on condi- 

 tions and background, with some advantage on 

 the side of white on black as being less ad- 

 versely affected by irradiation. A careful dis- 

 tinction should be drawn between the case 

 ihere considered of contrasted bodies returning 

 'light diffusely, and that of directed specular 

 reflection as from a mirror reflecting the sun. 

 This latter visibility, as in the observation of 

 a star, seems to depend chiefly on the min- 

 imum stimulus value for the retina under the 

 existing conditions of adaptation. Humboldt 

 ■records in his " Cosmos " the observation of a 

 heliograph mirror when subtending an angle 

 of only 0".43, and Professor Hosmer (M.I.T.) 

 tells me that his students could readily pick 

 up signals from a very small heliograph at 

 about 20 miles — angle subtended a scant 0".2. 



Some experiments by Barnard^ with a dark 

 wire 0".009 inch in diameter showed that it 

 was visible when suspended against mod- 

 erately bright sky up to 356 feet, angle sub- 

 tended 0".44. a figure down to something like 

 1/60 the diameter of the smallest spot or- 

 dinarily visible. 



iNagel, "Handbuch d. Physiologic d. Men- 

 schen," IIL, 340. 



2 Berl. Klin. Wocli., 1898, 20-22. 



3 Pop. Ast., 1898, p. 1. 



