Armand Masselin to Faraday   7 October 1859

Glass Works near Birmingham | October 7. 1859

Dear Sir

We beg to acknowledge receipt of your letter¹ concerning the proposed modifications we had lately mentioned in our letter to the Trinity House & will now answer your questions in the order you put them[.]

As regards the 16 upper & 6 Lower Catadioptric prisms forming the new Sections instead of the 12 upper & 5 Lower forming the old ones

“What is the Improvement Expected here[.] Is it in the greater accuracy of workmanship or the greater proportion of rays gathered in or in any other effect[.]” With a given unavoidable & practical inaccuracy of workmanship in any of the Surfaces or Curves, the Error becomes of a much lesser Consequence as the Size of prisms decrease because the Curves & the straight lines approach nearer[.] There is also a greater diminution of Substance of glass & therefore a lesser absorption of light & consequently also diminution of Color. | There is little or no difference in the proportion of rays of Light gathered in & the Strength of Construction of the apparatus is in no way whatever affected by the modifications.

By annular half Lenses we mean lenses having only about half the breadth of the Lenses of an apparatus mounted with 6 Sides[.] These lenses are Mounted in the same way, but in a narrower frame & Contain the Same number of rings, only the outer ones are Cut vertically[.]

Referring now to the number of lenses in the apparatus & to the speed of revolution We beg to State that,

“Then the second modification seems to be to construct the apparatus with 12 annular ½ Lenses & 12 panels of prisms instead of 6 or 8. You seem to consider 12 segments the best number for a light revolving at half minute intervals, but as I understand you only in relation to speed of motion Does the Expected improvement Consist in this difference in the Velocity of motion or is there any other effect that is contemplated because of the difference in the Size of the lenses.” A certain given ratio or speed of revolution is Supposed to be the most desirable & in order to increase flashes we prefer to increase the number of faces rather than the speed, or in other words prefer to decrease the total absolute power of each flash rather than to decrease the duration of the flash in relation to the eye[.] We must also beg to remark that the Same Size of Lamps being used, the divergence of the lenses will be the same in both the 12 & 6 sided arrangements only that with a 12 sided apparatus the duration of the flashes will be increased whilst the duration of the eclipse is shortened to the same extent. An apparatus with 8 sides could be made as well as an apparatus with 6 or 12 sides, but the 6 & the 12 are the usual divisions practically used[.] We beg to enclose in this letter 2 tracings giving the Sections of the apparatus the old 12 & 5 Sections as well as our new 16 & 6 Sections as recently Calculated by our Mr James T. Chance, which Sections may assist you in your decision[.]

We may also add that these Sections with increased numbers of prisms are against our interests as manufacturers as the expense of Constructing them increase nearly in proportion to the number of prisms, but we felt bound to give to the Trinity House all Suggestions in our Power[.]

Of Late years the first order Lights have been nearly all Constructed upon the new Sections Containing 18 & 8 prisms instead of 13 & 6[.]

We Remain Dear Sir | Yours faithfully | Chance Bros. & Co | Per A. Masselin

Professor Faraday &c &c.