Faraday to George Herbert   5 July 1861

[Royal Institution embossed letterhead] | 5 July 1861.

My dear Sir

As I shall not be able to see you tomorrow evening I write to acknowledge your note¹ with many thanks - The comet² offers a good illustration of the variation of quantity & intensity of light & their effect in producing sensation - As yet philosophers have discovered no difference in the effect in the eye as to mere visibility of the same light whether it be made diffuse or concentrated & intense i.e if the diffuseness be not carried too far. Thus two candle flames placed side by side near together or one behind the other are sensibly alike. Or a bats-wing or fish tail burner flame placed edge ways or sideways gives equal illumination at the same spot. Lights which are utterly imperceptible when alone must produce an effect on the eye long before they become sensibly visible[.] Thus a star of the 12th. or 14th. magnitude is not perceived by the eye. The eye cannot see the stars of the milky way as stars, without a powerful telescope but the sum of the effect of many of them together gives the milky way - which is a bright object[.]

Ever Truly Yours | M. Faraday

Geo Herbert Esqr | &c &c &c