March 24, 1887] 



NA TURE 



501 



outside our atmosphere, and at first blush the amount of extra 

 blue which he deduced to be present in it would, he thought, 

 make it so ; but though he surmised the result from experiments 

 made with rotating disks of coloured paper, he did not, I think, 

 try the method of using pure colours, and consequently, I 

 believe, sli^litly exaggerated the blueness which would result. 

 I have taken Prof. Langley's calculations of the increase of 

 intensity for the different rays, which I may say do not (juite 

 agree with mine, and I have prepared a mask which I can 

 place in the spectrum giving the different proportions of each ray 

 xs calculated by him, and this when placed in front of the spec- 

 trum will show you that the real colour of sunlight outside the 

 atmosphere, as calculated by Langley, ran scarcely be called 

 bluish. Alongside I place a patch of light which is very 

 closely the colour of sunlight on a July day at noon in England. [ 

 This comparison will enable you to gauge the blueness, and you i 

 will see that it is not very blue, and, in fact, not bluer per- i 

 ceptibly than that we have at the Kiffel, tlie colour of the 

 sunlight at which place I show in a similar way. I have also 

 prepared some screens to show you the value of sunlight after [ 

 l)assing through five and ten atmospheres. On an ordinary clear j 

 day you will see what a yellowness there is in the colour. It j 

 seems that after a certain amount of blue is present in white 

 light the addition of more makes but little difference in the tint. I 

 But these last patches show that the light which passes through 

 the atmosphere when it is feebly charged with particles does 

 not induce the red of the sun as seen through a fog. It only 

 requires more suspended particles in any thickness to induce it. 



In observations made at the Riffel, and at 14,000 feet, I have 

 found that it is possible to see far into the ultraviolet, and to 

 distinguish and measure lines in the sun's spectrum which can 

 ordinarily only be seen by the aid of a fluorescent eye-piece 

 or by means of photography. Circumstantial evidence tends 

 to show that the burning of the skin, which always takes place 

 in these high altitudes in sunlight, is due to the great increase 

 in the ultra-violet rays. It may be remarked that the same 

 kind of burning is effected by the electric arc light, which is 

 known to be very rich in these rays. 



Again, to use a homely phrase, ' ' You cannot eat your cake 

 and have it." Vou cannot have a large quantity of blue rays present 

 in your direct sunlight, and have a luminous blue sky. The latter 

 must always be light scattered from the former. Now, in the 

 high Alps you have, on a clear day, a deep blue-black sky, very 

 different indeed from the blue sky of Italy or of England ; and 

 as it is the sky which is the chief agent in lighting up the 

 shadows, not only in those regions do we have dark shadows 

 on account of no intervening — what I will call — mist, but 

 because the sky itself is so little luminous. In an artistic point 

 of Wew this is important. The warmth of an English landscape 

 in sunlight is due to the highest lights being yellowish, and to 

 the shadows being bluish from the sky-light illuminating them. 

 In the high Alps the high lights are colder, being bluer, and 

 the shadows are dark, and chiefly illuminated by reflected direct 

 sunlight. Those who have travelled abroad will know what the 

 effect is. A painting in the Alps, at any high elevation, is 

 rarely pleasing, although it may be true to Nature. It looks 

 cold, and somewhat harsh and blue. 



In London we are often favoured with easterly winds, 

 and these, unpleasant in other ways, are also destructive 

 of that portion of the sunlight which is the most chemically 

 active on living organisms. The sunlight composition of a 

 July day may, by the prevalence of an easterly wind, be reduced to 

 that of a November day, as I have proved by actual measurement. 

 In this case it is not the water particles which act as scatterers, 

 but the carbon particles from the smoke. 



Knowing, then, the cause of the change in the colour 

 of sunlight, we can make an artificial sunset, in which we 

 have an imitation light passing through increasing thicknesses 

 of air largely charged with water particles. [The image of a circu- 

 lar diaphragm placed in front of the electric light was thrown on 

 the screen in imitation of the sun, and a cell containing hypo- 

 sulphite of soda placed in the beam. Hydrochloric acid was 

 then added: as the fine particles of sulphur were formed, the 

 disk of light assumed a yellow tint, and as the decomposition 

 of the hyposulphite progressed, it assumed an orange and 

 finally a deep red tint.] With this experiment I terminate 

 my lecture, hoping that in some degree I have answered the 

 question I propounded at the outset : why the sun is red when 

 seen through a fog. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE 



Cambridge. — From the University accounts for i886, just 

 issued, we learn that the Mathematical Tripos Examiners are 

 [>aid 348/. ; Examiners in Natural Science, 575/. ; in Medicine 

 and Surgery, 210/. ; all extremely moderate payments. 



.Science Professors received 3725/. from the University Chest, 

 1800/. from the Common University Fund (derived from tax on 

 the Colleges), besides payments from special endowments ; 

 Readers, Demonstrators, and other officers connected with 

 Science and Medicine, 2100/. from the University Chest ; 1800/. 

 from the Common University Fund. Total, 9425/. for teachers 

 mainly. 



The University Observatory cost 786/., in addition to 164/. 

 from the Sheepshanks Fund. The Botanic Garden cost 1223/. ; 

 the Museums and Lecture Rooms, 4221/., including 100/. for 

 Dr. Guillemard's collection of bird-skins from the voyage of 

 the Marchesa, \\l. for bird-skins bought by Prof. Newton at the 

 Jardine sale, and 8/. \os. for a skeleton of a European elk. The 

 Pathological Laboratory cost 167/. out of the foregoing amount ; 

 the Department of Human Anatomy, 356/. ; the Woodwardian 

 Museum, 498/. ; the Chemical Laboratory, 517/. ; the Caven- 

 dish Laboratory, 274/., including 60/, for instruments. The 

 new dissecting-room (iron) for Human Anatomy cost 350/., an 

 additional class-room for Physiology 10/., charged to the 

 Museums Reserve Fund. 



At Gonville and Caius College, Dr. Shuttleworth's Scholar- 

 ship of 60/. for three years, open to medical students of the 

 University of not less than eight terms' standing, given for 

 proficiency in Botany and Comparative Anatomy, has been 

 awarded to Francis Henry Edgeworth, B.A., Scholar of the 

 College. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, March 3. — "On the Limiting Distance of 

 Speech by Telephone." By William Henry Preece, F. R.S. 



The law that determines the distance to which speaking by 

 telephone on land-lines is possible, is just the same as that which 

 determines the number of currents which can be transmitted 

 through a submarine cable in a second. 



It is dependent on a time-constant varying with the con- 

 ditions of the circuit, invariable for the same uniform circuit, 

 but differing for different circuits. It represents the time that 

 elapses from the instant contact is made at the sending-end to 

 the instant that the current begins to appear at the receiving- 

 end. It is given by the following equation : — 



a = Bi'rl'-, 

 B being a constant dependent principally on the units used ; k 

 the inductive capacity per unit length (mile or knot) ; r the 

 resistance per unit length, and / the lengths in miles or knots. 



The number of reversals which can be produced at the end of 

 a wire per second is quite independent of the impressed E. M. F. , 

 and therefore of the strength of the current. But it depends 

 upon the sensitiveness of the apparatus used to receive the 

 currents. This is why such discordant results are obtained by 

 different observers who attempt to measure the velocity of cur- 

 rents of electricity. It is also why the telephone is such an 

 admirable instrument for research — for it is sensitive to the least 

 increment or decrement of current. 



The inductive capacity of overhead and undergroim 1 wires 

 was measured with great care on very dry days in different parts 

 of the country. 



The results come out as follows : — 



Capacity Resistance 



per mile, per mile, 



microfarads B.A. ohms 



No. 7^ iron wire o-oi68 ... i2'o 



No. 1 2i copper wire 00124 ... 57 



Gutta-percha-covered wire in iron 



pipes 0-2500 ... 23-0 



Gutta-percha-covered wire in cables o"2900 ... lO'25 



It then became necessary to determine the speed of the current 

 through wires of different lengths, resistances, and capacities. 



It was found that, for mixed wires, the speed was given by 

 the equation 



/ := 32 X lO'* KR, 

 but for copper alone the constant was 22 x 10"". 



