46 



DISCOVERY 



direction, such as the synthetic production of phos- 

 phorescent substances, resembhng those met with in 

 nature in the firefly, which yields a hght without 

 perceptible heat, and is believed to attain an efficiency 

 approaching loo per cent. All the energy expended 

 by the firefly in the process appears as light. 



Artificial Daylight 



Another problem with which physicists are at pre- 

 sent struggling is the production of " artificial day- 

 light," namely an artificial illuminant yielding light 

 which in colour almost exactly resembles that received 

 from a white north sky. Such a source would prove of 

 inestimable value in many industries involving accurate 

 colour-matching processes. Lamps for use in this 

 way have recently been introduced, and it appears that 

 already natural daylight can be imitated with very fair 

 accuracv. But at present this can only be done by a 

 " subtractive " method. Light from a gas filled 

 electric lamp is modified, either by transmitting it 

 through a sheet of tinted glass (such as that intro- 

 duced by Chance Bros. & Co.) or by reflecting it off 

 a specially coloured surface (as in the Shcringham 

 Daylight). Such methods inevitably involve waste of 

 light, which may be as much as 60 per cent. Not- 

 withstanding this loss, such lighting units have con- 

 siderable value. But our ultimate aim should be to 

 produce directly light from all parts of the spectrum 

 to produce the desired " white light," so that no ex- 

 traneous modifying device, involving absorption, is 

 needed. 



enormous candle-power may be obtained. A lighthouse 

 recently installed on the French coast is credited with 

 emitting a light of 1,000,000,000 candle power. 



In the motor car headlight we have another instance 

 of judicious direction of light. The problem of combining 

 sufficient power of beam to reveal distant objects, 

 with absence of glare to approaching drivers or pedes- 

 trians, appears almost insoluble. But many ingenious 

 devices for confining most of the light below a hori- 

 zontal plane under the eye-level, and for distributing 

 it cvenlv along the road, have been introduced. 



The Kinematograph Projector 



In the kinematograph projector there is another 

 opportunity for the exercise of ingenuity by inventors. 

 Notwithstanding all that has been done to improve 

 the mechanism of the kinematograph, the projecting 

 apparatus is still very wasteful of light. It has been 

 estimated that only about i per cent, of the light 

 emitted by the arc actually reaches the screen. And 

 even the light which forms the picture is not all use- 

 fully applied, for much of it is reflected towards the 

 walls and ceiling and never meets the eyes of the 

 audience. 



REFERENXES 



Modem Illuminants end Illuminating Engineering. By L. 



Gaster and J. S. Dow. (Sir Isaac Pitman & Sons, Ltd.) 

 Artificial Light — its Influence upon CiinlisaHon. By M. 



lAickiesh. (University of London Press.) 

 The Official Organ of the Illuminating Engineering Society in 



London {Illuminating Engineer), and the Transactions of 



the Illuminating Engineering Society in the United States. 



Advances in Application of Artificial Light 



Hitherto we have spoken chiefly of advances in the 

 production of light. But advances in its application 

 have been equally important. These applications 

 have been made the subject of study by the Illuminat- 

 ing Engineering Society for the past fourteen years, and 

 we now understand much better how light should be 

 applied in the home, and in schools, streets, factories, 

 etc. In particular we have learned how to measure 

 illumination bymeansof convenient portable apparatus, 

 and thus to evolve standards for various purposes. 

 Great progress has been made in methods of directing 

 light. In some cases, e.g. in illuminating a picture 

 or " flood-lighting " the exterior of a building, we aim 

 at furnishing even illumination over a vertical surface. 

 At other times, in illimiinating, for example, the landing 

 area at an aerodrome, our object is to distribute light 

 over an extensive horizontal area. In the searchlight, 

 on the other hand, we have an entirely different prob- 

 lem, that of concentracing all the available light within 

 a ver}' small angle. Bv such concentration a beam of 



Between the Covers 



THE WOMEN OF GREENLAND AND THEIR 

 DRESS 



In our September and October numbers last year we 

 published two papers on Greenland by Professor 

 A. C. Seward, F.R.S., who made personal investigations 

 of the fossils and plant life of that Arctic continent 

 during the summer of 1 92 1. Professor Seward has- 

 now published a fuller account of his geological and 

 botanical investigations, together with a description 

 of his experiences, and impressions of the scenery and 

 the natives on the west coast, under the title of 

 ^4 Summcy in Greenland (Cambridge University Press, 

 7s.). He records some interesting facts concerning 

 the women of Greenland and their dress : " It is the 

 duty of Eskimo women to skin and cut up the seals, 

 and this is performed with wonderful dexterity by 



