WEEKLY EVENING MEETING,

Friday, June 1, 1866.

SIR HENRY HOLLAND, Bart. M.D. D.C.L. F.R.S. President, in the Chair.

HENRY E. ROSCOE, B.A. F.R.S.

On the Opalescence of the Atmosphere.

On a previous occasion (May 22, 1864) the speaker explained the principles of a method by the application of which we are able to gain some knowledge of the distribution of the chemically active rays on the earth's surface, and their variation from time to time. This method depends upon the comparison of tints gained by sensitive photographic paper when exposed to daylight; and it is evident that we must define the "chemical rays to be all those which are able to produce a darkening effect upon chloride of silver paper. In order that such a mode of measurement should be possible, it is necessary, in the first place, that paper can be prepared of a uniform degree of sensitiveness; and secondly, that the relation between the several tints and the intensity of the light necessary to produce such tints should be known. These relations have been accurately ascertained, and the method is now so far perfected that the observations can be very easily and accurately made.*

The whole apparatus needed for these experiments is contained in a sinall box, and all the observations for a day can be made in the course of a few minutes.

Through the kindness of Mr. Balfour Stewart, determinations of the Chemical Intensity of Total Daylight have been carried on at Kew Observatory for the past year by Mr. T. W. Baker. The mean daily intensity can be readily obtained from the separate observations, and these, when plotted out as a curve, show the daily mean intensities for the year.

Daily Mean Chemical Intensities measured at Kew, April, 1865, to April, 1866.

May
June
July
August
September,

In Spring, 45 9

In Summer, 91.5

In Autumn, 73.9 In Winter, 11.0 (Light of the Intensity 1 acting for 24 hours taken as 1000.)

See 'Phil. Trans.,' 1865, p. 605: "The Bakerian Lecture."

It is seen that the condition of the sky and weather materially influences the chemical intensity of the month-thus June, 1865, was cloudy (2 days rain; 21 days cloudy; average amount of cloud, 5 ̊5), and the mean intensity is 76 9; whereas September was a very bright month (0 days rain; 20 days cloudy; average amount of cloud, 2·5), and the chemical intensity reached 110 2.

If we compare the mean intensities for the summer and winter solstices and the equinoxes as measured at Owen's College, Manchester, we have,—

The above numbers show that the increase of chemical action from December to March is not nearly so great as that from March to June. This difference cannot be attributed to the common absorption exerted by the atmosphere, but may be explained as being the necessary consequence of a peculiar absorptive action which the atmosphere effects upon the chemically active rays, and to which the name of opalescence may be given.

It has frequently been stated that the chemical intensity of light on snowy peaks and in tropical climates is much less than that in our own latitudes, and that photographers in Mexico have found it impossible amidst the glaring rays of a tropical sun to obtain a picture which in the gloomy atmosphere of England would need an exposure of only one minute. In order to ascertain the degree of truth attaching to these extraordinary statements, and to obtain some insight into the chemical intensity of tropical climates, the speaker was fortunate to be able to send his assistant, Mr. T. E. Thorpe, to Pará, on the Amazons (long. 48° 30′ W., and lat. 1° 28' S.). The measurements there made have already furnished some very interesting results; in the first place, we find that the daily mean chemical intensities at Pará and at Kew on the same days are represented by the following numbers :

The curves for these days show the enormous variation of chemical intensity which occurs under a tropical sun during the rainy season. Every afternoon regularly, and sometimes at other periods of the day, the enormous thunder-clouds discharge their contents in the form of deluging rain, and the chemical action sinks to zero; then the storm passes over, and the chemical intensity again rises (see woodcut on opposite page). It is thus seen that any difficulties which a photographer may have in the tropics cannot be ascribed to an insufficient supply of the sun's chemically active rays.

* See Golding Bird, Natural Philosophy,' 5th edition, p. 622.

The speaker desired, however, chiefly to direct attention this evening to some experiments which appear to throw light upon that much vexed question of the cause of the blue colour of the heavens and the ruddy tints of sunrise and sunset. Since the time of Leonardo da Vinci this subject has been a favourite ground for the display of meteorological speculations. Leonardo, and afterwards Goethe, believed that the blueness of an unclouded sky was due to the passage of the

white light through the atmosphere containing finely-divided particles. Newton explained the blue colour of the heavens by the existence in the atmosphere of hollow very minute vesicles of water, upon which, as on a soap-bubble, the colours of thin plates become perceptible; and according as the thickness of the walls of these vesicles increased, so would the colour change from blue to yellow, orange, and red; and thus, by very frequent reflections, the various tints from skyblue to sunset-red could be explained. Founded upon this theory Clausius has calculated the relative intensities of direct sunlight and the diffuse reflected light of the sky for varying altitudes of the sun.

Some physicists have assumed that the air itself has a blue colour, whilst others have admitted that if air be of a blue colour by reflected light, it must appear red by transmitted light.

Others again, in order to avoid the difficulty of explaining the

great variety of sunset tints, have assumed these tints to be an ocular deception, or caused by the presence of clouds which receive and repeat the colour!

Many physicists have suggested that the atmosphere, being filled with small particles of floating solid matter, acts like an opalescent medium and transmits only red light; but it is to Brücke* that we are indebted for a complete statement and masterly investigation of this view of the subject. Forbes, again,† explains the phenomena in an entirely different manner; for he, observing that under certain circumstances aqueous vapour, or rather water in finely divided particles, is able to absorb the blue rays, and that the sun looked red when seen through a particular portion of a jet of escaping steam, attributes the sunset-red solely to the presence of water in this peculiar state of division.

In order to appreciate the value of these various opinions, it appears of special interest to obtain a knowledge of some quantitative facts respecting the intensity of the light transmitted directly from the sun, and that reflected by the air or particles in the atmosphere The possibility of making such measurements with respect to those parts of the sun's light which may be expected to show great differences in reflection and transmission, viz. the most refrangible portions, is rendered at once evident by the employment of the simple method of measuring the chemical intensity of light which has been above alluded to. The method employed consists simply in determining the chemical intensity of the total daylight (sunlight and diffused light), and immediately afterwards shading off the sun's direct rays by means of a small disc or sphere of metal, whose apparent diameter is only slightly greater than that of the solar disc seen from the position of the sensitive paper. In this way the chemical intensity of the total (direct and diffused) light is compared with that given off by the whole of the heavens alone, and the difference gives the chemical intensity of the direct sunlight.

Experiment soon proved that the relative intensity of the chemical light coming directly from the sun is very much less than we should ordinarily suppose, judging from the intensity of the visible light. Thus, at Owen's College, Manchester, it was found when the sun was 12° 3' above the horizon, that of 100 chemically active rays falling on the horizontal surface, less than 5 were due to the direct sunlight, whilst 95 came from the diffused light of the heavens, even when the sky was unclouded. At the same instant, of 100 rays of visible light as affecting the eye, 60 came directly from the sun, and only 40 from the diffuse sky-light. This singular result was also observed at Cheetham Hill, by Mr. Baxendell, and at Heidelberg, by Dr. Wolkoff; indeed, at this latter station, it was found on several occasions that whilst the sun was shining brightly, it was totally devoid of chemical rays, the

* Pogg. Ann.,' vol. lxxxviii. p. 363.

+ On the Colour of Steam under certain Circumstances, and on the Colours of the Atmosphere :" Edin. Transactions,' xiv. p. 371; ‘Phil. Mag.,' xiv. xv. 3rd Series.

interposition of the small disc producing no diminution in the chemical action.

Thus, at altitudes from 0° 34' to 12° 58' on the following occasions, the sunlight was robbed entirely of its chemically active rays by passage through the atmosphere.

The same inactive condition of the sun at low altitudes has frequently been observed at Kew, Cheetham Hill, and Owens College.

The following numbers give the results of an extended series of observations made at Heidelberg, by Dr. Wolkoff, at Kew by Mr. Baker, at Cheetham Hill by Mr. Baxendell, at Owens College by myself, and at Pará (Brazils) by Mr. T. E. Thorpe

The last column gives the ratio of chemical intensity of sun to sky, the fraction of the action of the diffuse light which the direct sun exerts. Thus, the ratio 0.106 at Owens College means that if 1 represents the intensity of the chemical light from the diffused light of the whole sky, 0106 was the intensity of the ray emanated directly from the sun.