out the North American ranges of the species. The casual records are added separately, but, we regret to see, with altogether too little specific data. The egg dates are generalized records 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 treatment of the questions of relationship, our author, however, occassionally adds comments of this character. One of the most interesting of these relates to that peculiar form Uria ringvia, which some ornithologists consider a distinct species, and others a mere aberration 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 recently 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 represent nearly twice that many scenes in the life of the various species, and consist of half tones showing habitat, nests, eggs, young, and sometimes 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 natural size, but there is, unfortunately, no indication 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 contributions to North American ornithology, and will for a long time be the recognized authority on biography of the species that it treats. HARRY C. OBERHOLSER

SPECIAL ARTICLES

VISIBILITY OF BRIGHT LINES

THERE has been a material amount of investigation 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 difficulty, be resolved when distant l', to judge from the average of many experiments,1 depending on relative contrast of the objects and other experimental conditions, and barring occasional cases of highly abnormal acuity, V=5-8, such as those reported by Cohn.2 A single spot, white on black or black on white can be detected by one with fairly keen vision down to a diameter of 30", by an occasional observer to half this value, again depending on conditions and background, with some advantage on the side of white on black as being less adversely affected by irradiation. A careful distinction should be drawn between the case here 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 minimum 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 Barnard3 with a dark wire 0.009 inch in diameter showed that it was visible when suspended against moderately bright sky up to 356 feet, angle subtended 0.44. a figure down to something like 1/60 the diameter of the smallest spot ordinarily visible.

1 Nagel, "Handbuch d. Physiologie d. Menschen," III., 340.

2 Berl. Klin. Woch., 1898, 20-22. 3 Pop. Ast., 1898, p. 1.

Later Lowell tried out a similar experiment with the result of finding his wire visible easily at 0.89, with some difficulty at 0.83 and glimpsed down to 0".69. Evidently his contrast conditions were less good than Barnard's. A further test by Slipher and Lampland showed the wire disappearing from certain vision at 0".86, while a dark blue line on a white disk held down to 0.83. W. H. Pickering experimenting with a dark human hair against open sky found it easily visible when subtending an angle of 1".13, easily glimpsed at 0".97, occasionally glimpsed at 0.83, and quite invisible at 0".72.

Taking up the converse case the writer first tried a German silver wire 0.01 inch diameter stretched zigzag in lengths of several feet over a dark plank bulkhead. The reflectivity of this varied from about 0.06 to 0.12, i. e., a very dark gray. The test was in full sunshine and the observers, the place being the range of the Massachusetts Rifle Association, were a group of riflemen, keen of sight and experienced in close observation. The terrain was laid off in 50-foot spaces and the results were as follows: Wire vanished across lighter parts of background at 75 feet (2".3) while across the darkest of that background the wire persisted up to 200-250 feet, beyond which it was invisible save for specular glints especially at twists. To summarize:

1′′.11..Parts against dark background were plain. 0".86.. Portions seem distinctly but not steadily. 0".69..Visible at specular spots, difficultly glimpsed elsewhere.

0".46..Visible by specular reflection only.

A second test with some of the same observers was made, using a background of black paper (coefficient .045), white thread 0.008 inch in diameter, and drawn tungsten wire 0.005 diameter. The paper was nailed to the former bulkhead and wire and thread stretched zigzag as before. Observed in bright skylight, also in moderately bright sunshine. The wire was vicible with difficulty to one

4 Bulletin Lowell Obs., No. 2.

5 Lowell Obs. Bull. No. 10.

6 Pop. Ast., 23, 578.

In the case of the thread the brightness contrast between thread and background was about 16:1. With brilliant sunshine and a background of even deader black there might have been a slight further gain, but we were evidently close to the limit. It is rather noteworthy that there should be so near an agreement throughout as between dark on bright and bright on dark, but barring specular direct reflection the brightness contrast which determines visibility is not widely different in the two cases, and the minimum visibile for a linear object with strongly contrasted background would appear to be about 0".5±. It is certainly less than 1/50 the minimum visibile for a round spot giving similar contrast, a remarkable evidence of the efficient coordination of retinal impressions. BOSTON

LOUIS BELL

tion, and of this we may be assured by a study of its proceedings in conjunction with the history of contemporary progress. Although the British Association can not claim any paramount prerogative in this good work, yet it can certainly claim to provide a free arena for discussion where in the past new theories in science, new propositions for beneficial change, new suggestions for casting aside fetters to the advancement in science, art, and economics have first seen the light of publication and discussion.

For more than half a century it has pleaded strongly for the advancement of science and its application to the arts. In the yearly volume for 1855 will be found a report in which it is stated that:

The objects for which the association was established have been carried out in three ways: First, by requisitioning and printing reports on the present state of different branches of science; secondly, by granting sums of money to small committees or individuals, to enable them to carry on new researches; and thirdly, by recommending the government to undertake expeditions of discovery, or to make grants of money for certain and national purposes, which were beyond the means of the association.

As a matter of fact it has, since its commencement, paid out of its own funds upwards of £80,000 in grants of this kind.

DEVELOPMENTS PRIOR TO THE WAR

It is twenty-nine years since an engineer, Sir Frederick Bramwell, occupied this chair and discoursed so charmingly on the great importance of the next-to-nothing, the importance of looking after little things which, in engineering, as in other walks of life, are often too lightly considered.

The advances in engineering during the last twenty years are too many and complex to allow of their description, however short, being included in one address, and,

following the example of some of my predecessors in this chair, I shall refer only to some of the most important features of this wide subject. I feel that I can not do better than begin by quoting from a speech made recently by Lord Inchcape, when speaking on the question of the nationalization of coal: "It is no exaggeration to say that coal has been the maker of modern Britain, and that those who discovered and developed the methods of working it have done more to determine the bent of British activities and the form of British society than all the Parliaments of the past hundred and twenty years."

James Watt.-No excuse is necessary for entering upon this theme, because this year marks the hundredth anniversary of the death of James Watt, and in reviewing the past it appears that England has gained her present proud position by her early enterprise and by the success of the Watt steam-engine which enabled her to become the first country to develop her resources in coal, and led to the establishment of her great manufactures and her immense mercantile marine.

The laws of steam which James Watt discovered are simply these: That the latent heat is nearly constant for different pressures within the ranges used in steamengines, and that, consequently, the greater the steam pressure and the greater the range of expansion, the greater will be the work obtained from a given amount of steam. Secondly, as may now seem to us obvious, that steam from its expansive force will rush into a vacuum. Having regard to the state of knowledge at the time, his conclusions appear to have been the result of close and patient reasoning by a mind endowed with extraordinary powers of insight into physical questions, and with the faculty of drawing sound practical conclusions from numerous experiments de

vised to throw light on the subject under investigation. His resource, courage and devotion were extraordinary.

In commencing his investigations on the steam-engine he soon discovered that there was a tremendous loss in the Newcomen engine, which he thought might be remedied. This was the loss caused by condensation of the steam on the cold metal walls of the cylinder. He first commenced by lining the walls with wood, a material of low thermal conductivity. Though this improved matters, he was not satisfied; his intuition probably told him that there should be some better solution of the problem, and doubtless he made many experi. ments before he realized that the true solution lay in a condenser separate from the cylinder of the engine. It is easy after discovery to say, "How obvious and how simple," but many of us here know how difficult is any step of advance when shrouded by unknown surroundings, and we can well appreciate the courage and the amount of investigation necessary before James Watt thought himself justified in trying the separate condenser. But to us. now, and to the youngest student who knows the laws of steam as formulated by Carnot, Joule and Kelvin, the separate condenser is the obvious means of constructing an economical condensing engine.

Watts experiments led him to a clear view of the great importance of securing as much expansion as possible in his engines. The materials and appliances for boiler and machine construction were at that time so undeveloped that steam pressures were practically limited to a few pounds above atmospheric pressure. The cylinders and pistons of his engines were not constructed with the facility and accuracy to which we are now accustomed, and chiefly for these reasons expansion ratios of from twofold to threefold were the usual prac

tise. Watt had given to the world an engine which consumed from five to seven pounds of coal per horse-power hour, or one-quarter of the fuel previously used by any engine. With this consumption of fuel its field under the conditions prevailing at the time was practically unlimited. What need was there, therefore, for commercial reasons, to endeavor still further to improve the engine at the risk of encountering fresh difficulties and greater commercial embarrassments? The course was rather for him and his partners to devote all their energy to extend the adoption of the engine as it stood, and this they did, and to the Watt engine, consuming from five to seven pounds of coal per horse-power, mankind owes the greatest permanent advances in material welfare recorded in history.

With secondary modifications, it was the prime mover in most general use for eighty years, i. e. until the middle of last century. It remained for others to carry the expansion of steam still further in the compound, triple, and, lastly, in the quadruple expansion engine, which is the most economical reciprocating engine of to-day.

Watt had considered the practicability of the turbine. He writes to his partner, Boulton, in 1784: "The whole success of the machine depends on the possibility of prodigious velocities. In short, without God makes it possible for things to move them one thousand feet per second, it can not do us much harm." The advance in tools of precision, and a clearer knowledge of the dynamics of rotating bodies, have now made the speeds mentioned by Watt feasible, and, indeed, common, everyday practise.

Turbines. The turbine of to-day carries the expansion of steam much further than has been found possible in any reciprocating engine, and owing to this property it