Faraday to Jacob Herbert   24 June 1841

Royal Institution | 24th June 1841.

Sir,

Having examined the eight French Refractors¹ now at Mr. Wilkin's Manufactory, I beg leave to make the following general report upon the degree of perfection to which, in optical workmanship and construction, they have been carried.

When an object, as a light, is placed in the principal Focus or Centre of the arrangement, each rib ought theoretically, to cause the refracted and issuing rays to proceed nearly parallel to any distance. If they converge and cross, the point at which they cross may be called the secondary Focus, and the nearer it is to the Refractor the worse is the workmanship. I could only examine these Refractors to a distance of 30 feet:- but of the 136 ribs, of which the eight Refractors consist, only 20 fall nearer than 30 feet, 116 being beyond.

I have already reported to the Board on the English Refractors²;- but for the sake of comparison will give the summary of the seven Gibraltar Refractors and the spare one accompanying them, the numbers being 51 to 58. These consist of eighteen ribs each or 144 in the whole, out of which there are but ten that have their secondary foci beyond 27 feet:- 134 being within that distance. Nay more; 127 are less than 20 feet, and as many as 116 are less than ten feet.

The rays that issue from the different ribs of the French refractors are exceedingly well adjusted to each other, and, with a focus of 36 or 36 1/2 inches, approach well to parallelism. There are some ribs whose rays cross the other rays within a distance of 20 feet, but they are not numerous, nor are they principal ribs.

In consequence of this degree of perfection in workmanship, when these refractors are examined by the parallel rays of the sun, they exhibit a very fine effect;- i.e. the place indicated for the proper place of the flame is nearly 36 inches as it ought to be, and the line of the light there produced is compact and close, instead of being wide and scattered, and has much burning power.

The colour of the glass of the French Refractors is very good generally:- much surpassing that of the English Refractors.

I have a record of the results of a minute examination relating to the measured distances of the foci, the direction of the rays &c:- but have thought a general report sufficient in the present instance. I have open'd two of the boxes containing the mirrors, but not understanding the numbers attached to them, I cannot make out their intended places and distance. I was therefore fearful of disturbing them and as they can be examined hereafter, have, for the present left them undisturbed³.

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