1890.] Photometric Observations of the Sun and Sky. 5 



of measure of chemical action the darkening produced at a distance 

 of 1 inch from the wick of the candle when 100 grains were con- 

 sumed, which in the candle I used in India occupied about forty- 

 seven minutes. My observations, being almost entirely relative, are 

 independent of these assumptions, which affect hardly any of my 

 results except comparisons with the absolute unit measures of Sir 

 H. Roscoe. 



5. Explains the method by which I obtain a standard strip for the 

 candle unit. 



7. Describes the water motion actinometer, with which observations 

 of the action of sun and sky were made. 



8. Shows how it may be proved experimentally that the intensity 

 of the action of light emanating from a physical point varies in- 

 versely as the square of the distance from the origin. 



9. For obtaining the effects of the sun and sky, I have always ex- 

 perimented mainly by exposing the paper at right angles to the sun's 

 rays. Sir H. Roscoe, on the other hand, exposes his paper on a 

 horizontal plane. Theoretic considerations have led me to another 

 method of observation (with the " octant " actinometer below) which 

 gives directly the measure of the. effect really desired. 



A table is given of the first observations I made, which afterwards 

 led to the formation of Table B (see next page). 



11. The method of observing the action of the sun alone. 



12. Observations taken near the horizon not to be depended upon. 



13. Refers to the construction of Table B, and the extension of the 

 table for altitudes of the sun beyond those observed. 



14. Shows how the numbers of the table were obtained, by taking 

 the inverse of the times required at each altitude for producing the 

 darkening of the candle unit. 



17. I found the chemical action of the sun, as far as my experiments 

 went, the same at all hours of the day and at all seasons of the year. 

 And in Somersetshire I got exactly the same chemical action of the 

 sun as at Dacca. 



18. Various observations had led me to suspect that the chemical 

 action of the sky at the same moment was diverse in different parts 

 of it. To investigate this suspicion, I designed an instrument which 

 I call the Mitrailleuse Actinometer (fig. 2). I mount a number of 

 similar cylindrical tubes in one plane in a semicircle, to the centre of 

 which the axis of each tube is directed : one extremity of each tube 

 lies in the circumference of the circle ; the other extremities lie on 

 a concentric circle of about one-half the radius. In the circumference 

 of this smaller circle is a semicircular series of holes, against which 

 a semicircular block carrying the sensitised paper is pressed by a 

 screw. . Each cylinder cuts out of the sky a circle of 8 28' angular 

 diameter. One of the tubes near its top carries a small plate of wood 



