Royal Society, 221 



chanically, not once, but for several known but varying intervals 

 of time quickly succeeding each other ; so that whatever may be 

 the intensity of the total daylight (supposed during these intervals 

 to remain constant), some one at least of the several exposed 

 papers will possess the requisite shade. This is accomplished by 

 a duplicate arrangement of a clock and insolation-apparatus, by 

 means of which disks of the constant sensitive paper are exposed 

 each hour for successive known intervals of time, varying from two 

 to thirty seconds. After an interval of an hour, another set of 

 disks are exposed for the same series of intervals ; and these series 

 of insolations are repeated once every hour during the day. The 

 mechanical arrangements for effecting this with accuracy are fully 

 described in the paper. On unrolling, at the end of the day, the 

 strip of sensitive paper which has served for the exposures, black 

 disks showing where the paper has been stationary for the hour 

 are seen ; and between each of these are found ten circles variously 

 tinted, from that, probably, scarcely visible, which was exposed for 

 two seconds, to that, perhaps too dark to read off, which was inso- 

 lated for thirty seconds. Amongst these, some one at least, will 

 be found of such a shade as to enable it to be read off by the mo- 

 nochromatic soda-flame, on a graduated fixed strip, as described in 

 former communications. 



A new method of calibrating the fixed strips of standard tints 

 necessary for these measurements is next described ; and the ques- 

 tion as to the possibility of preparing constant sensitive paper in 

 long strips instead of in large sheets is next experimentally dis- 

 cussed, the result of the examination being that it is possible to 

 prepare silvered paper in long narrow strips such as are used in 

 Morse's telegraph-apparatus, so that it shall throughout its length 

 preserve the standard sensitiveness. 



The time during which the disks of constant sensitive paper are 

 exposed is next ascertained for each instrument by a chronograph. 



During wet weather the insolator is covered by a semicircular 

 glass shade ; and the value of the coefficients for refraction and 

 absorption due to this glass shade is determined. 



The latter portion of the communication contains the results of a 

 series of comparisons of the curves of daily chemical intensity ob- 

 tained (1) with the hand-insolator, and (2) with the self-recording 

 instrument. Comparisons of this nature were made during the 

 months of May, June, and July, 1873, by simultaneous hourly de- 

 terminations in the neighbourhood of Manchester according to both 

 methods. Of these observations, six full days are selected ; and the 

 tables and curves accompanying the communication show the close 

 correspondence of both sets of observations. The integrals of total 

 chemical intensity for these days are also given, and exhibit as close 

 an agreement as, from the nature of the experiments, can be ex- 

 pected. 



