Faraday to Frederick Madan   2 September 1842

R Institution | 2 Septr. 1842

My dear Sir

The practical tables you refer to¹ include I dare say the effect of refraction and that effect is visibly to raise the horizon[.] Hence I rather think the principal cause of the difference between my number of about 30' & their number of 22' 1/2[.] I left the refraction out because I wished to approach as near as I could to your larger quantity of 1˚. But no doubt the visible depression is what concerns us just now.

You make the enquiry in your note whether it would not be better to put the lens out 1/8 of an inch at the top & as much in at the bottom? Now this without either elevating the lamp or lowering the lenses will do nothing but still leave the beam horizontal. On the other hand if you raise the lamp or lower the lenses keeping the latter perpendicular you will do all that is required[.] In fact to depress the beam 1˚ as you propose the lamp must be raised 0.63 or nearly two thirds of an inch or else the lenses lowered as much. Now as to the lenses you cannot well do that without interfering with the whole structure of the frame. Raising the lamp therefore seems the only practical mode[.] If after that you decide to tilt the lenses so that they should be perpendicular with respect to the ray impinging on their centres (and the lenses be as I think they are 30 inches high) then they should be rather more than half an inch out of the perpendicular i.e. 0.26 of inch outwards at the top & 0.26 of an inch inwards at the bottom but this is not particularly necessary to the practical effect.

I am My dear Sir | Your faithful Servant | M. Faraday

Capt F. Madan | &c &c &c