Faraday to Jacob Herbert   11 March 1842

Royal Institution | 11th March 1842

Sir,

You will probably remember that you desired me to ascertain, as nearly as I could, what proportion of light was intercepted by French Glass, Cookson's Glass, and Flint Glass respectively; and that you placed in my hands, a fine cube of French Glass, and endeavoured also to procure a corresponding cube of Cookson's Glass, or at least a sample of equal thickness in one direction. As, however, some time has elapsed without our being able to obtain such a specimen, I have lately had a piece cut off from a broken rib of a refractor from Cookson's manufactory, equal in length to the dimensions of the cube; and compared these one with the other in their action upon a ray of light. The French glass is of a pale blue colour and has some striae; Cookson's glass is of a deeper and greener colour and has striae, much stronger than in the French glass; it is, however, I think a fair sample of the glass in the refractors. Both the specimens are 2 1/2 inches in thickness in the direction through which the ray was passed. Both accompany this Letter.

It will be unnecessary for me to describe minutely the way in which, to the best of my judgement for practical purposes, a standard lamp was compared with another lamp, from which the light sometimes proceeded unobstructed, and sometimes passed through the one or the other of the two specimens of glass. Three different persons observed the results, for the purpose of doing away with any peculiarity in the vision, or any tendency in the mind, or a particular person; and numerous experiments were made the average of those for each glass being finally taken.

The result was that if a naked light had an illuminating power of .... 100.

it was reduced by the French Glass to .............................................................82 2/3

and by the Cookson's Glass to ..........................................................................67 1/3

so that the French Glass cut off rather more than a sixth of the whole light, and the Cookson's glass about a third.

This loss of light is partly due to the reflexion of the two surfaces of each piece of glass, and partly to the obscuring effect of the body of the glass; and the proportionate effect of the two causes it is very difficult to distinguish. The loss by reflection is probably nearly the same for both the glasses; but that by extinction, due to the colour and imperfect transparency, differs with the two. With reference to this, however, I must observe that the amount of effect given above is produced by as much as 2 1/2 inches of glass, whereas in the refractors the thickest part of the glass is not much more than half this amount, and from that it diminishes gradually 'till in other parts it is not more that a fifth or sixth of this quantity.

With reference to the action of striae, it is probable that these do not interfere as much as may at first be supposed. They disturb the course of the ray and spread light about, which otherwise would go on to a particular spot; but when the striae occur in different parts of the glass and in different ribs, the dispersion of one part covers the dispersion of another, and the general amount of light at any given spot, where an observer may stand, is very nearly the same as if there were no striae. This is just what was found on a much larger scale in the comparison of French and English refractors; where the ribs in the one were truly worked and placed, and in the other were not so. The first was a much more beautiful and accurate instrument when examined minutely, than the second; but in the broad and general effect of a Lighthouse, the errors in the different ribs of the second so far compensated each other, as to throw very little less light on to a given spot on the horizon, than was done by the perfect instrument. So it is with striae, to a considerable degree; for each deranged rib in a refractor may be considered as the same thing to the whole of that refractor as a striae in a piece of glass, is to the whole of the glass¹.

I have the honor to be | Sir, | Your obedient humble Servant | (signed) M. Faraday.

Jacob Herbert Esq | Secretary | &c &c &c | Trinity House