NATURE [January 12, 1893 



In the occasional use of supposed scientific principles 

 as a means of accounting for facts of animal structure. 

 He has dealt with a subject of great difficulty with 

 commendable clearness, and will interest readers who 

 would be unable to follow a more technical exposition of 

 -extinct types of life. H. G. S. 



ENERGY AND VISION. 



THE interesting researches of Prof. S. P. Langley on 

 energy and vision have recently been published in 

 the Memoirs of the American National Acaderriy of 

 Sciences. From this we gather that he was led to in- 

 vestigate the question by the fact that it was not generally 

 recognized how totally different effects may be produced 

 by the same amount of energy in different parts of the 

 spectrum. Two series of experiments were necessary, 

 the first to determine the amount of energy in each ray, 

 the second to observe the corresponding visual effect. 

 The energy was determined as heat by the use of the 

 bolometer, the heat dispersed by a prism being very 

 nearly proportionate to the energy. 



In the second series of experiments a beam of sun- 

 light from a siderostat passes -through a small hole in a 

 darkened room and falls on a slit with a standard width 

 of 01 mm. It is then received on a coUimating lens of 

 1 1 "9 centimetres aperture and 755 centimetres focal length, 

 after which it passes through a prism of about 60" re- 

 fracting angle. The spectrum thus formed is reflected 

 and brought to a focus on a second slit of one millimetre 

 aperture by a concave mirror, any particular colour being 

 adjusted on the slit by a rotation of the prism. This 

 second slit is screened from all possible stray light by 

 a dark curtain, and is used as a source of illumination for 

 a series of numbers from a table of logarithms, which is 

 attached to a shding screen. The greatest distance from 

 the slit at which the figures could be distinctly read was 

 then determined, and the law of inverse squares applied. 

 For the brighter colours of the spectrum, the light enter- 

 ing the first slit was reduced by an adjustable photometer 

 wheel. 



Actinometric measures were made during the progress 

 of the photometric observations, and showed a solar 

 radiation of i"5 calories per square centimetre per minute ; 

 this naturally being an essential unit. 



The energy necessary to give the bare impression of 

 luminosity in different parts of the spectrum, expressed 

 in terms of horse-power, was found to be roughly as 

 follows, the minimum visibile being defined as the feeblest 

 light which is observed to vanish and reappear when 

 silently occulted and restored without the knowledge of 

 the observer : — 



Horse-power. 



Violet (A 400) ... o'oooooo 000000 00018000 



Green (A. 550) ... o 000000 000000 00000075 



Scarlet (a 650) ... 0"oooooo 000000 00017000 



Crimson (A 750) ... O'oooooo 000000 34000000 



These values were derived from observations made by 

 a single observer, Mr. F. W. Very, and are, of course, 

 subject to a large percentage of error. 



The general results of the investigation may be best 

 summarized in Prof. Langley's own words : — 



" The time required for the distinct perception of an 

 excessively faint light is about one-half second. A re- 

 latively very long time is, however, needed for the re- 

 covery of sensitiveness after exposure to a bright light, 

 and the time demanded for this restoration of complete 

 visual power appears to be greatest when the light to be 

 perceived is of a violet colour. The amount of energy 

 required to make us see varies enormously according to 

 the colour of the light in question. It varies considerably 

 •between eyes which may ordinarily be called normal ones, 

 t'but an average from those of four persons gives the 

 NO. I 2 I I , VOL. 47] 



following proportionate result for seven points in the 

 normal spectrum, whose wave-lengths correspond ap- 

 proximately with those of the ordinary colour divisions, 

 where unity is the amount of energy required to make us 

 see light in the extreme red of the spectrum near A, and 

 where the six preceding wave-lengths given correspond 

 approximately to the six colours, violet, blue, green, yellow, 

 orange, red. 



Colour I Violet I Blue I Green I 



Wave length 400 470 530 



Luminosity | 1600 | 62,000 | 100,000 | 



Yellow I Orange 1 Red I 



580 60) 650 



28,000 I 14 001 I 1200 I 



Crimson 

 750 



It appears from this that the same amount of energy 

 may produce at least 100,000 times the visual effect in one 

 colour of the spectrum that it does in another. 



If now it be inquired what the actual value of unity is 

 in ordinary measure, we are able to give this also with a 

 fair approximation, and to say that the -Vis-viva of the 

 waves whose length is 7500 (tenth mecres) being arrested 

 by the ordinary retina, represents work done in giving 

 rise to the sensation of the deepest red light of about 

 o 001 of an erg in one-half second. 



NOTES. 



The Prince of Wales has consented to become Chairman of 

 the Committee for the memorial of the late Sir Richard Owen, 

 and to preside at a meeting to further the object, which will be 

 held in the rooms of the Royal Society, Burlington House, on 

 Saturday, the 21st inst., at half-past eleven o'clock. Admission 

 will be by tickets, which may be obtained from Mr. Percy 

 Sladen, Linnean Society, Burlington House, W. (who is acting 

 as secretary to the Committee), or from Mr. H. Rix, assistant 

 secretary of the Royal Society. 



The annual general meeting of the Royal Meteorological 

 Society will be held at 25, Gre at George-street, Westminster, 

 on Wednesday, the i8th instant, at 7.15 p.m., when the Report 

 of the Council will be read, the election of officers and council 

 for the ensuing year will take place, and the President (Dr. C. 

 Theodore Williams) will deliver an address on "The High 

 Altitudes of Colorado and their Climates," which will be illus- 

 trated by a number of lantern slides. This meeting will be 

 preceded by an ordinary meeting, which will begin at 

 7 p.m. 



The general meeting of the Association for the Improvement 

 of Geometrical Teaching is to be held at University College, 

 Gower Street, W.C., on Saturday, January 14, the Rev. C. 

 Taylor in the chair. At the morning sitting (il a.m.) 

 the report of the Council will be read, the new officers will be 

 elected, and several candidates will be proposed for election as 

 members of the Association. After the conclusion of the formal 

 business Mis. Bryant will give "A Model Lesson on 

 Geometry, as a Basis for Discussion." After an adjournment 

 for luncheon at i p.m. members will re-assemble (2 p.m.) to 

 hear papers by Mr. G, Heppel on "The Use of History 

 in Teaching Mathematics," and Mr. F. E. Marshall on 

 "The Teaching of Elementary Arithmetic." Members who 

 wish to have any special matter brought forward at the general 

 meeting, but who are unable to attend, are requested to com- 

 municate with one of the Honorary Secretaries. All interested 

 in the objects of the Association are invited to attend. 



Dr. Ludwig Becker has been appointed to the chair of 

 astronomy at the University of Glasgow. 



The Comet Medal of the Astronomical Society of the 

 Pacific Coast has been awarded to Mr. Edwin Holmes, of Lon- 

 don, for his discovery of a new comet on November 6. 



On Tuesday next (January 17) Prof. Victor Horsley, F.R.S., 

 will begin a course of ten lectures, at the Royal Institution, on 

 " The Functions of the Cerebellum and the Elementary Prin- 



