Faraday to John Hall Gladstone   2 July 1859

[Royal Institution embossed letterhead] | 2 July 1859.

My dear Gladstone

Although I have frequently observed lights from the sea the only thing I have learnt in relation to their relative brilliancy is that the average of a very great number of observations would be required for the attainment of a moderate approximation to truth. One has to be some miles off at sea or else the observation is not made in the chief ray & then one does not know the state of the atmosphere about a given lighthouse[.] Strong lights like that of Cape Gri[s]nez have been invisible when it should have been strong[;] feeble lights by comparison have risen up in force when one might have expected them to be relatively weak - and after enquiry has not shewn a state of the air at the lighthouse explaining such differences.- It is probable that the cause of difference often exists at sea -

Besides these difficulties there is that other great one of not seeing the two lights to be compared, in the field of view at the same time & same distance. If the eye has to turn 90° from one to the other I have no confidence in the comparison. And if both be in the field of sight at once still unexpected & unexplained causes of difference occur. The two lights at the South Foreland are beautifully situated for comparison & yet sometimes the upper did not equal the lower when it ought to have surpassed it. This I referred at the time to an Upper Stratum of haze - but on shore they knew nothing of the kind nor had any such or other reason to expect particular effects.

Ever Truly Yours | M. Faraday

Dr Gladstone | &c &c &c