Faraday to George Herbert   25 November 1852

Brighton 25, Novr. 1852

My dear Sir

Your figures¹ are I think all right but as they are founded on my own, which as I have said in the letter containing them² are quite insufficient for any serious conclusions, so of course they must partake of their imperfection; and I have no confidence in the results wherever they may be opposed to or want the confirmation of observation. There are too many data necessarily left out. For instance we do not know correctly the comparative loss of light by reflection from the silver and transmission & reflexion by the glass. The loss by silver has been thought to be the greatest but recently it has been found to be only one tenth of the whole quantity³ a result which Lord Rosse appears to have confirmed⁴:- Another point is that the oil that is burnt must be burnt in or near the focus to be useful; if burnt out of it it only gives as the result a larger divergence but not a brighter ray:- The divergence in any arrangement is chiefly dependant on the extent of the place of combustion[.] Another point is that the burner obstructs the progress of a very large portion of the light that is produced in or near the focus. In the four wicked lamp probably one half of the flame is hidden from the bottom of the refractor or the lens by the great breadth of the burner. It was this circumstance which to a large extent rendered the Gurney oxy lamp so deficient in effect: the light was intense but it was set down low upon the burner & much of it thus stopped in its course. Again in assuring a divergence of 6˚ or 15˚ the area cannot for a moment be supposed to be uniformly illuminated & the light in the axis of the beam does not represent the intensity of the light generally. Neither does it represent a given ratio to the area. I expect that the light in the axis of a good French lens is far more intense in relation to the light generally over the 6˚ of divergence than the light in the axis of a reflector is to the general light over its 15˚ of divergence[.] The extent of the divergence of 6˚ & 15˚ is only a general assumption approaching to the truth; and the progressive disposition or distribution of the light over the areas so represented is I believe even less clearly known to us than the extent of divergence. The brightness of the revolving lens light coexists with briefness the dullness of a single reflector with comparative duration and I suppose that this difference of duration which may be said to be as 6 to 15 must be of much importance in the character of a given light[.]

So after all, & as always, I think that observation is the only useful way of arriving at a good practical conclusion[.] It was in 1842 [sic] I believe that the experiments at Purfleet & Blackwall were made⁵. A first order French refractor - a first order English refractor and a reflector were compared and the results were such as to make me very cautious in drawing conclusions except from practice: for the English refractor, though not at all comparable to the French refractor in workmanship & the form of the ribs, was but little inferior to it in power & that chiefly because of the colour of the glass. Further the Parabolic reflector clearly surpassed both. I think your father⁶ was there with the Deputy Master⁷ & the Elder Brethren but at all events he has some account of the results.

I am away from home and my bad memory causes that I cannot give Wilkins results as to oil burnt⁸. That same bad memory makes me desire to preserve papers on which I have ventured an opinion even though the opinion be private. So though I send you your paper⁹ with my marginal notes, yet I will thank you to let me have both back again (you can copy my notes if you like) that I may store them away in their place.

Ever My dear Sir | Very Truly Yours | M. Faraday

Geo Herbert Esq | &c &c