1864.] Prof. Roscoe^ — Registration of Action of Daylight. 557 



giving the results obtained in several series of determinations, in order that 

 the amount of experimental error may be estimated. Curves exhibiting 

 the graduation of several strips are also given ; and from these the author 

 concludes that the determinations agree as well as can be expected from 

 such photometric experiments, the mean error between the positions 40 

 and 80 min. on the strip in one series of graduations not exceeding 1 per 

 cent, of the measured intensity. To each fixed strip a Table is attached, 

 giving the intensity of the light which must act for 1 second upon the 

 standard paper, in order to produce the tints at each millimetre of the 

 length of the strip. 



The methods of exposure and reading are next described. The exposure 

 of the paper is effected very simply by pasting pieces of standard sensitive 

 paper upon an insolation band, and inserting the band into a thin metal 

 slide having a small opening at the top and furnished with a cover, which 

 can be made instantly to open or close the hole imder which the sensitive 

 paper is placed. When one observation has thus been made, and the time 

 and duration of the insolation noted, the remaining papers can be similarly 

 exposed at any required time ; and thus the determinations can be very 

 easily carried on at short intervals throughout the day. 



The reading-instrument consists of a small metallic drum, furnished 

 with a millimetre scale, and upon which the graduated strip is fastened. 

 The drum turns upon a horizontal axis, and the insolation band, with the 

 exposed papers upon it, is held against the graduated strip, so that by 

 moving the drum on its horizontal axis the various shades of the strip are 

 made to pass and repass each of the papers on the insolation band, and the 

 points of coincidence of tint on the strip and on each of the exposed 

 papers can be easily ascertained by reading off with the monochromatic 

 soda-flame. 



In the next section of the paper the author investigates the accuracy 

 and trustworthiness of the method. This is tested in the first place by 

 making simultaneous measurements of the chemical action of daylight by 

 the new method and by means of the pendulum photometer, according to 

 the mode described in the last memoir, upon which the new method is 

 founded. Duplicate determinations of the varying chemical intensity 

 thus made every half-hour on four separate days give results which agree 

 closely with each other, as is seen by reference to the Tables and figures 

 showing the curves of daily chemical intensity which are given in the 

 paper. Hence the author concludes that the unavoidable experimental 

 errors arising from graduation, exposure, and reading are not of sufiicient 

 magnitude materially to affect the accuracy of the measurement. As a 

 second test of the trustworthiness and availability of the method for actual 

 measurement, the author gives results of determinations made with two 

 instruments independently by two observers at the same time, and on the 

 same spot. The tabulated results thus obtained serve as a fair sample of 

 the accuracy with which the actual measurement can be carried out ; and 



