Photometric Observations of the Sun and Sky. 257 



47. minutes. [I am here narrating the course I pursued in com- 

 mencing these observations at Dacca ; I very soon discarded the 

 candle, as I was able, by the aid of my table given below, to recover 

 the unit of measurement by a Sun observation.] 



5. I form a strip of photographic paper about f inch deep into a 

 circular ring, placed inside a metal cylinder 3 inches in diameter. I 

 place now my standard candle eccentrically, at a distance of I inch 

 from the surface, and burn the 100 grains of stearine. I thus get a 

 strip which is gradually coloured from the point nearest the centre 

 (where the intensity is unit i) to the most remote point (where the 

 intensity is j i). By calculating the distances of various points of 

 the ring from the wick, the intensities corresponding to these dis- 

 tances can be marked. I exhibit a small strip (of somewhat dif- 

 ferent dimensions) so calibrated to show a ocale of intensities ; it 

 has lost its original shade in consequence of fixing and toning. 

 For actual purposes of measurement, a strip is used in its original 

 unfixed state. 



6. My earlier observations on the chemical action of the Sun and 

 sky, were made in Bengal, with a " mica actinometer." In this, small 

 squares of one sheet of sensitised paper were covered by 1, 2, 3, 4 

 .... thicknesses of mica cut from the same plate ; the sheet of 

 paper then exposed to any light for a certain time gave me a series 

 of chequered shades. To measure the effect of the Sun or of any 

 portion of the sky, I noted the time necessary to darken the paper 

 till it matched one of the squares in blackness. This instrument I 

 have long since laid aside, as I have superseded it by better ; but by 

 its aid in 1863 I was led to the attempt of measuring the chemical 

 action of the Sun, in a clear sky, for each degree of the Sun's altitude, 

 so as to form a table of constants, which would render a direct 

 reference to the candle power unnecessary. 



7. I have made an instrument (fig. 1) similar to one employed in 

 India. The plane on which to expose the sensitised paper has 



iotions in altitude and azimuth ; a perpendicular style is placed at 

 corner ; and, by shifting the plane until the style casts no shadow, 

 ie plane can be adjusted at right angles to the Sun's rays, and the 

 Sun's altitude can be read by a brass Gunter's quadrant. A slide 

 lich covers the strip of sensitised paper, is made to move uniformly 

 the plane, by means of a string passing over a pulley attached to a 

 it in a column of water in a long cylinder (the one used in India 

 was a rain-gauge) ; the float descends as the water is drawn off by a 

 stopcock at the bottom of the cylinder. Lines can be drawn on a 

 gauge pasted on the plane, beside the longitudinal slit, in which is 



t posed the sensitised paper, corresponding to the motion for 1, 2, 3, 

 . . 20 seconds ; also a second gauge has been drawn for a larger 

 p giving quicker motion. By simply moving the sensitised paper 



