Faraday to Robert Gordon   24 October 1859

Royal Institution | 24 October 1859

To the Deputy Master | of the Trinity House | &c &c &c

You have done me the honour to request that I would communicate to you such observations as arose in my mind in relation the special points which came before us during the late visit to the coast of France¹ (i.e. to Calais, Cape Grisnez and Boulogne) these points being the pile light house at the Point du Walle;- the use of paraffine in a light house lamp;- and the effect of a sound bell placed in a reflector :- I will now endeavour to meet your desire.

The screw pile light house placed on the edge of the sands at the point du Walle is supplied with an apparatus which is nearly of the third order having a double wicked lamp burning Colza oil. The intention is to produce a continuous light characterized by varying from the red to white and white to red. Viewed at night from the Pier at Calais about 6 miles off it had this character; and as far as I could judge the intensity of the red light & the white light were about equal to the eye.

The manner in which this effect is obtained is as follows. Consider a horizontal plane a as passing through the focus or middle of the flame of the lamp; it will divide the light into two portions, the upper & the lower hemispheres, of which the lower will be the weaker because the burner interferes & cuts off part of the light proceeding towards the dioptric apparatus around. From this horizontal plane downwards the lamp is surrounded by fixed dioptric pannels b & below them by fixed catadioptric reflectors c which refract & reflect the light upwards and collect it into the horizontal plane; so that as far as that part of the apparatus is conceived the light is a fixed light all round. On the same fixed apparatus and above the usual place of the pannels are the catadioptric reflectors d, and these also add to the constant white light from the lower part for they also send their rays in a horizontal direction all round[.] Thus the white light which is seen from a distance is obtained.

The part above the horizontal plane a & below the catadioptric reflectors d is diverted to the production of red light. Suppose a lens placed in the usual way before a lamp; it would collect all the rays falling on to it into a horizontal beam, and if fixed on to a revolving frame would give flashes:- But if, without disturbing the position of the lens, it were cut in half by the horizontal line a & the lower half removed, then only rays from the upper hemisphere would be collected in the flash & the lower rays would be left to go through the prism pannels b. If the lower half of the lens cut off were turned bottom upwards & placed above the horizontal line, it would act just as the half lens left undisturbed; and there are, indeed, eight such half lenses e filling up the space round the circle and fixed on to a revolving frame so as to give eight flashes in one revolution of the apparatus.

If the apparatus were used in this state it would give a constant white light from the fixed part of the dioptric arrangement to which would be super added the flashes of white light from the moving lenses; but the object is to convert these flashes into red light and this is done in the following manner. The moving frame carries 24 panes of red glass. Eight of these are placed before the eight half lenses; they occupy the whole of the circle & convert what would be white flashes into red flashes. These red flashes would however be mingled with white light from the lower & upper fixed apparatus which would increase the light but diminish the red colour; So to convert this white light into red light, whilst the flash from the lenses is passing to the observer, a pane of red glass for each lens descends from the moving frame before the lower glass, and another ascends from it before the upper catadioptric reflectors; but their width is only enough to subtend an angle of 4° or 5°, by which however they convert the white constant light into red whilst the red lens is sending its rays to the observer thus adding red light from the fixed pannels to the red light from the moving lenses[.] So the alternate red & white lights are produced[.]

As the visible effect at Calais Pier of the two lights was to me nearly equal, it gives occasion to remark on the serious loss of light produced by any attempt to colour it effectually. In the present case when the white light appears it comes from parts of the apparatus which are equally open to view when the red light is on; but the narrow red panes are interposed and only the red light came through this being however greatly increased because at that moment the light from the lenses is brought in aid. As there are only eight lenses round the circle, & the ray from each may be considered as having a divergence of 7° or 8° (for the lenses are of moulded glass) it is probable that if all the red glass were away they would give conjointly with the light from the fixed apparatus a beam of white light 4 or 5 times as strong as that which appears in the white beam of the apparatus as it is now arranged; yet, being reddened by the interposed glass that beam was not at all to my view stronger in illumination[.]

I may say here that I understood M. Reynauds strong desire to establish a distinctive character by change in the colour of the light is greatly founded upon the tendency mariners have to confound alternating lights of different intervals with each other. Rotating lights with quarter & half minute intervals have been mistaken for those of half & whole minutes, & vice versa; the tendency is, to observe & distinguish a rotating light from a fixed light; but to neglect the time, & so lose the distinction between one & another.

All the glass of this apparatus has been moulded instead of being cut & wrought to shape by machinery. However the inner surface of the pieces & some of the larger surfaces of the outer part, have been wrought. The moulded glass did duty; but it could not be properly examined in its present position; & as I expect to have specimens in my hands for particular investigation, I will say no more on the matter at present.

Paraffine or the oil of schest is a fluid very rich in carbon; often applied in lamps economically; & yielding a very beautiful light if carefully burned. When carelessly burned it produces much smoke with a bad flame. It has been applied to the Harbour light at Boulogne and appears to answer there perfectly. The flame was very regular and, by the deflector, was expanded horizontally until its width was an inch & a half, whilst its vertical height was not more than half an inch. This shape is necessary for the perfect combustion, & is the best shape also for lighthouse use; a tall narrow flame being just as objectionable as the one described is advantageous. The lamp has but one wick & this substance cannot be burnt in lamps with two or more concentric wicks, the necessary deflector being then inapplicable to them. The substance may probably come into use for Harbour lights but scarcely for lights of a superior order[.]

Fog bell. A bell weighing perhaps 3 cwt has been fixed up at the end of the western pier of Boulogne harbour. It is placed in a species of reflector consisting of Iron with a regular face of cement; & is furnished with three hammers & a mechanical apparatus by which it can be stuck in times of need. It was to have been sounded as we left the harbour; but in place of that, a smaller open bell was rung; & by reason of the wind & waves its sound was lost at a very small distance outside. Whilst experimenting on the pier head with the bell in the reflector, I found that there were great interferences in the strength & direction of the sound. The sound was best right in front;- passing right or left in a circle at the same distance from the bell, the sound fell & was a minimum when about 45° had been gone through. Going round still further it rose again in intensity until the edges of the reflector interfered, & then as one moved behind the bell, it fell to the lowest degree. It will require more & extended observation before any advantage the arrangement may possess can be tried & known.

Two first order lights came under our observation that at Calais and that at Cape Grinez. Each of these lights has a four wicked lamp and the oil is worked by pumps. I am satisfied that the fourth or central wick is an advantage. When the flame, which ought never to be more than 3½ inches high & always without smoking tongues, is perfect this central wick gives as clear & definite a flame as the others. The draught of the chimneys is powerful. The cottons are thin. The oil overflows abundantly:- not more than one fourth of that which is raised is burned the rest returns to the cistern, but in overflowing, preserves the cottons in excellent condition. The cottons in fact rise out of a burner of oil - The consumption of oil is from 750 10875gr to 800 grammes 12347gr (1.32 pints) per hour and that is well burned.

The light at Calais is a fixed light with the occurrence of a flash every four minutes[.] It consists, first of all, of a fixed catadioptric apparatus which produces the constant light;- then outside of this, is a moveable frame, carrying three dioptric pannels, equidistant from each other, & which, as they refract in a horizontal plane, produce with the former apparatus the effect of lenses. This frame revolves in 12 minutes & this produces the final result. The motion is very slow & easy; but the arrangement, though it has no objection in principle, is not in favour, & is the only one in France. The Grinez light consists of 16 lenses which revolve; above & below there are the old captoptric [sic] reflectors which give a constant light. The lens frame revolves in 8 minutes, so that there is a flash every half minute; and the time of its duration is estimated at 8 seconds.

A consideration often occurs in respect of revolving lights, whether it is most important to make a strong quick flash or a weaker longer one? It is easy to see, that, if there were only 8 lenses in the Grinez circle & the whole revolved in 4 minutes, that there would be an equal number of flashes in the same time, i.e one every half minute; and that the flash would be of double the strength for it would contain twice the light gathered by the half sized lens;- but, then, this greater flash would only endure 4 seconds instead of 8. Now the impression of M. Reynaud & the other authorities, is, that increase in time up to a certain amount is more important than increase in brightness; and that 8 seconds is the least time during which the flash ought to endure. So strong is this impression, that, where in some cases the reflectors above & below the lenses have been made holophotal, they have not been so placed as to add their power to that of the lenses so as to increase their flash in brightness, but are so arranged as to precede by a little the flash of the lenses, & so increase it in duration[.] This is an important question which can only be settled by much observation[.]

The South Foreland Electric light: As far as I heard & saw by the reports all the observers on the coast of France opposite report well of the power & constancy of the Electric light at this place[.]

I do not think there is any other matter on which I need speak²[.]

I have the honour to be | Sir | Your Very faithful humble Servant | M. Faraday