172 



NA TURE 



[December 9, 1909 



done in training skilled wireless operators both in the 

 Navy and for the mercantile marine work. Radio- 

 telegraphy, like aviation, is an art as well as a science, 

 hence personal skill is a factor of importance in turning 

 the flank of the difficulties of the moment. Nevertheless, 

 the art and the science of radio-telegraphy are both pro- 

 gressing, and the splendid services already rendered by it 

 in saving life at sea are at once a proof of present perfec- 

 tion and an evidence that the arduous labours of investi- 

 gators and inventors have borne fruit in yet larger powers 

 to command the great forces of nature for the use and 

 benefit of mankind. 



ILLUMINATING ENGINEERINGA 



'T'HIS society has been founded to bring together all 

 those who are interested in the problems, practical 

 and theoretical, of the art of directing and adapting light, 

 that prime necessity of civihsed, as well as of uncivilised, 

 existence, to the use and convenience of man. To advance 

 the subject of illuminating engineering, to investigate 

 through all its lengthened breadth the facts within its 

 domain, to increase and diffuse knowledge respecting them, 

 and to unite those who are devoting their energies to 

 these things, is the object of the society. The ascertained 

 facts are few — all too few ; their significance is immense ; 

 their economic and social value is great ; but the ignorance 

 respecting them generally is colossal 



For practically a century only have there been any 

 systematic means of illumination in use in any civilised 

 country. Before the year iSon there were as means of 

 illumination daylight, oil lamps, rush lights, tallow dips, 

 and wax candles. Monarch and peasant, merchant-prince 

 and workman, had alike to depend on individual sources 

 of light at night. Only in the larger towns and cities was 

 there any organised attempt to light the streets by oil 

 lamps. In 1819 the authorities of the day stoutly resisted 

 the proposal to light the then House of Commons by gas — 

 nothing but wax candles could be admitted ; but gas light- 

 ing was coming in, and Argand and colza oil lamps were 

 the sole competitors until after 1850. Everything else 

 dates since then — practically during the last half-century. 

 For parafTm lamps were not widely spread until the 'sixties. 

 Arc lighting, though tried for spectacular and lighthouse 

 purposes from the 'fifties, did not come into public ques- 

 tion until about 1879. Glow-lamps followed three or four 

 years later. Still later came incandescent gas mantles and 

 acetylene gas lights, while the newest things in both gas 

 lighting and electric lighting are affairs of only a year or 

 two ago. .Many persons now realise the immense stride 

 made in the introduction of the Auer (Welsbach) mantle 

 for incandescent gas ; very many fewer people realise the 

 significance of the corresponding step forward that has 

 been begun by the introduction of the metallic filament 

 glow-lamp. We are on both sides in the very middle of 

 an immense evolution in the art of illumination. 



But whilst the means of illumination have thus been 

 developing with amazing strides during a single generation, 

 and the organised systems of distribution by municipal 

 and urban and rural authorities, and by private corpora- 

 tions, have ramified throughout the community and brought 

 supplies of gas and of electricity— shall I also' say of oil?— 

 to our doors, there has been another and very different 

 development going on. I refer to the growth of that 

 branch of the science of optics which deals with the 

 measurement of luminous values. Photometry has been 

 growing into an exact science by the explanation of its 

 laws and the improvement of the instruments of measure- 

 ment. It was not until 1760 that the first real discussion 

 of photometric principles was made known. In that year 

 Lambert, in his " Photometria," laid down the funda- 

 mental laws, and likewise in the same vear Bonguer gave 

 to the world his " Trait(5 d'Optique," wherein a primitive 

 photometer was described. Rumford's shadow photometer 

 was invented in 1794, and Ritchie's in 1824. Then comes 

 a long gap. Save for Bunsen's over-rated grease-spot 

 mstrument, there was no important advance in photometry 



1 Abridged from the inaugural address delivered at the inaugural meetin^ 

 of the Illuminating Engineering Society held on November -18, by Prof 

 Silvannv P Th ,„„ c- I, .■ president of the society. 



Silvanus P. Thompson, f.R. 



NO. 2093, VOL. 82] 



until the 'eighties, when there were produced many novel 

 forms, some of them, including the now well-known forms 

 of L. Weber, Lummer-Brodhun, and Rood, capable of 

 yielding results of much higher precision in the comparison 

 of different sources of light ; also in the 'eighties we meet 

 for the first time with special forms of photometer of the 

 kind destined to play a very important part in the work 

 of our society, many photometers measuring the values, 

 not of the brilliancy of a source of light, but the illumina- 

 tion of a surface. 



Our primary concern is the adequate and proper illumina- 

 tion of things ; and as we have to reduce the present chaos 

 to an exact science, our first business is to secure some 

 common agreement as to the measurement of illumination 

 and the establishment of reasonable rules as to the amounts 

 of illumination required in different cases. 



Foremost, then, in the prograinme of work for our 

 society we put the question of the units of measurements 

 and the promulgation of the proper definitions of them. 

 We must secure agreement — national and, if possible, inter- 

 national — as to what shall be taken as the unit of light 

 and what as the unit of illumination at a surface. 



Happily, the long-standing controversy as to the former 

 appears to be settling itself by at least a preliminary agree- 

 ment between the standardising laboratories of the great 

 nations. One " candle " is no longer to be a vague and 

 indefinite thing. The new definition provisionally agreed 

 upon is an ideal unit, in terms of which one can describe 

 the several standards in use in different countries. If this 

 provisional entente can but be ratified by a little inter- 

 national common sense, we shall have henceforward an 

 international " candle " such that it is the same in England 

 as in America, equal to the bougie decimale accepted in 

 France, and related to the Hefner-candle of Germany in 

 the precise proportion of ten to nine. 



But we have still to find agreement on the standard of 

 illumination. Here in England, and in the United States, 

 we have already grown accustomed to describe amounts 

 of illumination of surfaces in terms of a British unit — the 

 " candle-foot " — not perhaps a very happy term — one that 

 we would readily exchange for a better — meaning, thereby, 

 the intensity of the illumination at a surface situated 'at 

 the distance of one foot from a light of one "candle." 

 The source being assumed here to be concentrated at a 

 point, the law of inverse squares holds good. 



Adopting the candle-foot as the unit of Illumination, one 

 may readily state certain facts with definiteness. .\11 com- 

 petent authorities are agreed that at night, for the purpose 

 of reading, an illumination is required not less than one 

 candle-foot, some authorities saying i^ candle-foot. The 

 facts appear to be that reading is impossible with an 

 illumination of one-tenth candle-foot, dlflicult and fatiguing 

 with one of one-fifth candle-foot, comfortable with from 

 1 5 to 3 or 4 candle-foot, but that if the illumination exceeds 

 6 or 8 candle-foot, the glare of the page Is again fatiguing 

 and dazzling. The page should neither be under- 

 illuminated nor over-illuminated. Something depends. It is 

 true, on the size of the print. Under a feeble Illumination 

 of, say, i candle-foot, a type of pica size printed in a 

 fount of bold face properly inked is legible when one of 

 long-primer size, printed In a weak way, would be prac- 

 tically illegible. Something also depends on the state of 

 the eye as affected bv the general Illumination of the 

 surroundings. Verv seldom does one find In any ordinary 

 room an artificial illumination exceeding 3 candle-foot. 

 By day, on a writing-table placed near a north window — 

 or near any window not receiving direct sunlight — the 

 illumination may exceed 3, and may even attain 4 or 5 

 candle-foot. 



Until a unit of illumination was thus agreed upon, it 

 was impossible to render any reasonable certainty to 

 estimates of the amount of illumination in any case of 

 dispute. What is the meaning of the term well-lit as 

 applied to any room, building, factory, workshop, or 

 school? Formerly the term was entirely vague. To-day 

 the answer can be given in numerical terms. Formerly 

 judgment had to be made by the unaided eye, and the eye 

 is notoriously a bad judge. As between two different 

 illuminations, the powers of discrimination of the eye are 

 very limited. The eye can equate, but it cannot appraise. 

 It can tell with fair accuracy whether iwo adjacent patches 



