300 Sir W. cle W. Abney. On the Variation in 



attain their maxima values in 1839 and 1873, and those of the 

 westerly winds in April in 1837 and 1869. As the secular variations 

 of the sunspots have their maxima in 1837*2 and 1870*8, the agree- 

 ment is in close accord. 



There seems little doubt that, during the interval of time covered 

 by the present investigation, the meteorological phenomena, number of 

 aurorse, and magnetic storms, show secular variations of a period of 

 about thirty-five years, the epochs of which harmonise with those of 

 the secular variation of sunspots. 



As we are now approaching another maximum of sunspots which 

 should correspond with that of 1870*8, it will be interesting to observe 

 whether all the solar, meteorological, and magnetic phenomena of that 

 period will be repeated. 



Conclusion. 



1. There is an alternate increase and decrease in the length of a 

 sunspot period reckoning from minimum to minimum. 



2. The epoch of maximum varies regularly with respect to the pre- 

 ceding minimum. 



The amplitude of this variation about the mean position is about 

 ± 0-8 year. 



The cycle of this variation is about thirty-five years. 



3. The total spotted area included between any two consecutive 

 minima varies regularly. 



The cycle of this variation is about thirty-five years. 



4. There is no indication of the fifty-five-year period as suggested 

 byP>r. Wolf. 



5. The climate variations indicated by Professor Bruckner are 

 generally in accordance with the thirty-five-year period. 



6. The frequency of aurorse and magnetic storms since 1833 show 

 indications of a secular period of thirty-five years. 



"On the Variation in Gradation of a Developed Photographic 

 Image when impressed by Monochromatic Light of Different 

 Wave-lengths." By Sir William de W. Abney, K.C.B., 

 D.C.L., D.Sc, FES. Eeceived March 26— Eead May 2, 

 1901. 



Introductory. 



When a series of small spaces on a photographic plate are exposed 

 to a constant light for geometrically increasing times, or for a constant 

 time to geometrically increasing intensity of illumination, the spaces 

 so exposed will on development show deposits of silver of different 



