500 



NATURE 



{March 24, 1887 



is much felt. [Photographs showing these dififerences were 

 shown.] Let us ask to what this is due. In such chmates as 

 England there is always a certain amount of moisture present in 

 the atmosphere, and this moisture may be present as very minute 

 particles of water — so minute indeed that they will not sink down 

 in an atmosphere of normal density — or as vapour. When 

 present as vapour the air is much moie transparent, and it is 

 a common expression to use, that when distant hills look "so 

 close " rain may be e.tpected shortly to follow, since the water 

 is present in a state to precipitate in larger particles ; but when 

 present as small particles of water the hills look very distant, 

 owing to what we may call the haze between us and them. 

 In recent weeks everyone has been able to see very multiplied 

 effects of such haze. The ends of long streets, for instance, 

 have been scarcely visible though the sun may have been 

 shining, and at night the long vistas of gas lamps have shown 

 light having an increasing redness as they became more distant. 

 Everyone admits the presence of mist on these occasions, and 

 this mist must be merely a collection of intangible and very 

 minute particles of suspended water. In a distant landscape we 

 have simply the same or a smaller quantity of street-mist occupy- 

 ing, instead of perhaps looo yards, ten times that distance. Now 

 I would ask, What effect would such a mist have upon the light 

 of the sun which shone through it ? 



It is not in the bounds of present possibility to get outside our 

 atmosphere and measure by the plan I have described to you the 

 different illuminating values of the different rays, but this we 

 can do : — First, we can measure these values at different altitudes 

 of the sun, and this means measuring the effect on each ray after 

 passing through different thicknesses of the atmosphere, either at 

 different times of day, or at different times of the year, about the 

 same hour. Second, by taking the instrument up to some such 

 elevation as that to which Langley took his bolometer at Mount 

 Whitney, and so to leave the densest part of the atmosphere 

 below us. Now, I have adopted both these plans. For more 

 than a year I have taken measurements of sunlight in my labor- 

 atory at South Kensington, and I have also taken the instru- 

 ment up to 8000 feet high in the Alp-, and made observations 

 there, and with a result which is satisfactory in that both sets of 

 observations show that the law which holds with artificially 

 turbid media is under ordinary circumstances obeyed by sunlight 

 in passing through our air: which is, you will remember, that 

 more of the red is transmitted than of the violet, the amount 

 of each depending on the wave-length. The luminosity of the 

 spectrum observed at the Rilfel I have used as my standard 

 luminosity, and compared all others with it. The result for four 

 days you see in the diagram. 



I have diagrammatically shown the amount of different 

 colours which penetrated on the same days, taking the Riffel as 

 ten. It will be seen that on December 23 we have really 

 very Httle violet and less than half the green, although we have 

 four-fifths of the red. 



The next diagram before you shows the minimum loss of light 

 which I have observed for different air thicknesses. On the top 

 we have the calculated intensities of the different rays outside 

 our atmosphere. Thus we have that through one atmosphere, 

 and two, three, and four ; and you will see what enormous 

 absorption there is in the blue end at four atmospheres. The areas 

 of these curves, which give the total luminosity of the light, are 

 761, 662, 577, 503, and 439 ; and if observed as astronomers 

 observe the absorption of light, by means of stellar observations, 

 they would have had the values, 761, 664, 578, 504, and 439 — a 

 very close approximation one to the other. 



Next notice in the diagram that the top of the curve gradually 

 inclines to go to the red end of the spectrum as you get the light 

 transmitted through more and more air, and I should like to show 

 you that this is the case in a laboratory experiment. Taking a 

 slide with a wide and long slot in it, a portion is occupied by 

 a right-angled prism, one of the angles of 45' being towards the 

 centre of the slot. By sliding this prism in front of the spec- 

 trum I can deflect outwards any portion of the spectram I like, 

 and by a mirror can reflect it through a second lens, forming a 

 patch of light on the screen overlapping the patch of light formed 

 by the undeflected rays. If the two patches be exactly equal, 

 white light is formed. Now, by placing a rod as before in front 

 of the patch, I have two coloured stripes in a white field, and 

 though the background remains of the same intensity of white, 

 the intensities of the two stripes can be altered by moving the 

 right-angled prism through the spectrum. The two stripes are 



now apparently equally luminous, and I see the point of equality 

 is where the edge of the right-angled prism is in the green. 

 Placing a narrow cell filled with our turbid medium in front of 

 the slit, I find that the equality is disturbed, and I have to allow 

 more of the yellow to come into the patch formed by the blue 

 end of the spectrum, and consequently less of it in the red end. 

 I again est.ablish equality. Placing a thicker cell in front, 

 equality is again disturbed, and I have to have less yellow still 

 in the red half, and more in the blue half. I now remove the 

 cell, and the inequality of luminosity is still more glaring. 

 This shows, then, that the rays of maximum luminosity must 

 travel towards the red as the thickness of the turbid medium 

 is increased. 



The observations at Sooo feet, here recorrled, were taken on 



September 15 at noon, and of course in latitude 46° the sun could 

 not be overhead, but had to traverse what would be almost 

 exactly equivalent to the atmosphere at sea-level. It is much 

 nearer the calculated intensity for no atmosphere intervening, 

 than it is for one atmosphere. The explanation of this is easy. 

 The air is denser at sea-level than at 8000 feet up, and the 

 lower stratum is more likely to hold small water particles or dust 

 in suspension than is the higher. 



For, however small the particles may be, they will have a greater 

 tendency to sink in a rare air than in a denser one, and less 

 water vapour can be held per cubic foot. Looking, then, from my 



Fig. 3. — Proportions of Transmitted Colours. 



laboratory at South Kensington, we have to look through a pro- 

 portionately larger quantity of suspended particles than we have 

 at a high altitude when the air thicknesses are the same ; and 

 consequently the absorption is proportionately greater at sea-level 

 that at 8000 feet high. This leads us to the fact that the real 

 intensity of illumination of the different rays outside the atmo- 

 sphere is greater than it is calculated from observations near sea- 

 level. Prof. Langley, in this theatre, in a remarkable and 

 interesting lecture, in which he described his journey up Mount 

 Whitney to about 12,000 feet, told us that the sun was really blue 



