Faraday report to Trinity House   16 August 1860

Report, &c. To the Deputy-Master¹ & Brethren of the Trinity House

The Royal Commission on Lights &c, having desired to meet the Brethren of the Trinity house at the North Foreland and Whitby Lighthouses, with certain other persons, such as the manufacturers of the apparatus, the representatives of the Irish and Scotch lighthouses, the Astronomer Royal, &c, for the purpose of making certain practical communications to them which could not be so well done by writing or elsewhere; such meetings have taken place. The one at the North Foreland occurred on the 2nd of August; when there were present Adml Hamilton, - Captn Ryder, Dr. Gladstone, the Astronomer Royal, and Mr. Campbell, of the Royal Commission;- Adml Gordon, Captn Close, Captn Baily², [blank in MS]³ of the Trinity House; Sir James Dumbrain [sic]⁴ of the Irish Board⁵;- Mr. Stevenson of the Scotch Board; Mr. Sautter of Paris, the maker of the apparatus with Mr. Wilkins;- and myself;- the other at Whitby occurred on the 9th of the same month; when the same persons were present, with the exception of the Astronomer Royal, Captn Baily, & Sir James Dumbrain; and the addition of Mr Halpin⁶ of the Irish Ballast Board, and⁷ Mr. J. Chance, the manufacturer of the apparatus, with M. Masselin. I was there on the part of Trinity house, as their Scientific adviser in Experimental lights; and though I do not wish to assume the character or responsibility of Optical or Civil engineer, I feel it my duty, at the call of the Deputy Master⁸, to give my impression and conclusions. Having received no descriptive written document from the Royal Commission, I may perhaps pass by some points requiring attention, unawares.

The object of the R Commission was, I believe, to point out the necessity of a final examination of the optic apparatus in the lighthouse itself, after its erection, and the demonstration of a mode of making that examination in a practical manner, which I may call the method of the Commission. The object the optic apparatus in a lighthouse is to convey the rays from the source of light to the sea horizon, or to the parts of the sea between that horizon and the shore, in the most abundant degree possible & in the most favourable manner. This effect is obtained by a certain position & adjustment of the parts of the refracting & reflecting apparatus and whether these are right in this respect, may be ascertained by a process in some degree the reverse of the illuminating method. Thus; suppose the lamp is lighted, a given bright spot in its flame selected, and a given piece of the glass apparatus adjusted, so as to transmit the light passing through it from the chosen spot to the sea horizon:- then, if the adjustment be right, an observer placed on the other side of the flame, & looking through the spot and the glass, will see the horizon; & this is an observation easily made in the daytime, either with the flame actually existing, or with indicating gauges at the burner (as the edge of a card for instance) representing given or selected parts of the flame. If the horizon does not appear in the right place the adjustment is wrong. The same kind of test may be applied to every part of the apparatus in turn, whether refractive or reflective;- in respect of any part of the horizon, or the sea, or the sky; and in respect of any part of the flame or luminous object. A luminous point being selected, that place which the eye sees through it, will be the place to which the rays issuing from it in that direction, will proceed.

This appears to me to be an excellent practical application. In principle it is perfect. The application however cannot be equally perfect because of the inevitable imperfections of construction. Still the quality of the glass and its workmanship have arrived at such a degree of excellence, as to justify the application of this refined kind of inspection; which may be anticipated & worked up to, by processes applicable in the manufactory.

Assuming that the apparatus is as perfect in its execution as can be expected, then the causes that may interfere with due effect, (and chiefly by mal-adjustment) are several. First, those connected with the lamp & flame. The Fresnel lamp has a burner & cottons having a horizontal width of 3¾ of an inch in diameter;- the flame as it rises from this base is obscure above & near to the cotton, then becomes luminous and powerful, and, contracting as it rises, usually ends in a series of forky tongues. When supplied by an overflowing lamp urged by the draught of a good chimney, the bright part of the flame may be from 2½ to 4 inches from the bottom to the top of the chief tongues of flame; these latter having no smoke at their tops; and the horizontal section passing through the widest and brightest part of the flame will be from 6/8 to 9/8 of an inch above the burner. It is assumed that this section should coincide with the focal plane of the lenticular bands, or merely refracting part of the apparatus; for then the most powerful rays proceed in a horizontal direction, & will fall on the sea horizon, when the light has little or no elevation above the sea. All the light which emanates below that plane & passes through the lenticular bands, will be thrown up into the sky above the horizon; but all that, emanating from the great body of the flame above that plane, will be cast over the sea between the horizon & the shore, doing good service to the mariner.

The selection of this plane, or of the point in the centre of the flame coinciding with it, which is called the focal point in respect of the refractors, is of great importance. By numerous experiments & trials in France, it is considered as 28mm. or 1.05 of an inch above the burner. Now the brightest horizontal section of the flame, may be 1.12 inches above the burner with a well arranged overflowing lamp; or only 0.75 of an inch above it, with a low flame & non overflowing lamp;- & the adjustment in height of the lamp which would suit the first, & send a fine body of light from the upper part of the flame over the sea, would, with the latter, send little to the sea & the greater part of the light to the sky:- so that not merely has the adjustment to be attended to, but also the lamp suited to the adjustment.

As far as my observation goes, the lamp should overflow freely, so that only one fourth of the oil that passes over & through the wicks, should be burnt:- it should, in the case of a first order lamp, have four wicks;- and a chimney of glass & iron 6 feet high;- As much oil as possible should be burnt without smoking; for when in a good state, the light is as the oil burnt. The lights at Whitby had not overflow lamps; and only a certain amount of oil could be burnt & a certain height of flame (lower at the South than at the North lighthouse) be obtained. By making the oil overflow & raising the wick, the flame was raised at the base, but began to smoke;- by virtually lengthening the chimney, through the application of paper valves, the whole flame was raised, both at the base & at the summit, and the combustion very greatly improved, & that continuously. But the adjustment of the optical part, fit for the one state on flame would not be fit for the other. Being examined in the manner proposed by the Commission, if the combustion were low, the ray proceeding from the eye to the horizon would be much too high in the flame; whereas with a full & proper flame, it might pass in the best direction; for the difference in level of the brightest sections of two such flames may be as much as ⅜ of an inch;- and every diminution in the good condition of the flame, whether from the construction of the lamp, or inattention, tends to rob the flame at the upper of sea supplying part.

In respect of the reflectors circumstances are different. The whole of the flame radiates light towards each of the upper reflectors. It the eye be so placed (as in the R Commission process) as to see the horizon through the flame, in the middle of one of the reflecting prisms, then all the flame above that line, will throw its light into the sky, and only that part below the line will throw its light on to the waters. Hence, the line through the flame, or rather through its projection at the reflector, should go through a bright & abundant part of it, and should also leave as much as possible of the flame below that line; since that is the part which radiates light to the sea i.e the observers ray should be taken as far back towards him, & as far up, as is consistent with a good line of flame for the horizon; and then the reflector ought to be adjusted, so as to throw this light which has reached it onward in the right direction[.] As regards the upper reflectors, generally, a point in the centre of the flame, 1.55 inches above the burner, is, from careful experiment considered as that which gives the best result; and is called the focal point for the upper reflectors, being common to all.

In respect of the lower reflectors, matters are very different: The burner & cottons cut off much of the light of the flame from them. With the best flame one half of the light is thus lost; & with a low flame, only a fourth or a fifth may pass to them to be utilized. The line of sight should, as regards the observer, be taken as far forward & up, as is consistent with its passing through a bright part of the flame; for here again, it is the part of the flame below this, which sends light to the sea, whilst the part above it casts its rays into the sky. In the French experience and practice, not one focal point, but several foci are taken for the different reflecting prisms. These are points in a vertical line, in the centre of the flame; the lowest focus is for the upper reflector & the highest for the lower reflector; & they are respectively at the following heights above the burner 38, 42, 47, 53, 60 & 68 millimetres[.]

The lamp, optical apparatus, & adjustments, I have referred to, have had reference to a horizontal line; and it is so, that nearly all the apparatus made in, or for England, have as yet been so constructed; but the sea horizon does not correspond with a line horizontal at the lighthouse;- it forms an angle with it, and that so much the greater as the light is higher above the level of the sea. At the North Foreland the two make an angle of about 14'.2, and at Whitby of about 16'.5. Hence if the chief ray of light be sent horizontally it will pass over the sea & be wasted, and indeed more light with it, even the 16'.5 seconds, which ought to fall on the sea. This condition is seen at the Whitby lighthouses by the Commissioners mode of examination a little modified. To correct this error for the lenticular bands, it would be sufficient to raise the lamp an equal number of minutes, as 1/6 of an inch for Whitby; but such a proceeding would increase the error for the reflectors, both above and below; and can only properly be met by instructions to the maker of the apparatus at first.

The French authorities, only take account of this difference between the Sea & true horizon, when the height of the light is 60 meters (about 200 feet) & upwards, above the Sea. For my own part I do not see why it should not be taken into account for any height of 50 feet and upwards. Twelve or thirteen years ago the Lundy light had its chief rays sent to the Sea horizon, & I made an instrument which was used by Mr Wilkins for the proper adjustment of the reflectors⁹. Since then the reflectors have been changed for others on the catadioptric principle, & these have their rays directed horizontally as at first, & the same rule has held ever since. Mr. Chance tells me he is now constructing apparatus with the rays directed to the sea horizon[.]

Cases may arise where high light, not being a leading light, might better have its chief ray sent, not to the extreme sea horizon, but some intermediate distance, where in hazy weather, the light might require to be in some degree concentrated[.] Such cases are nautical in their nature; but if they occur, instructions should be given to the maker before hand; since the correction or adjustment can not properly be made afterwards[.]

The first application of a new and searching method of examination not applied until after the workmen have been fitting & handling the heavy parts of the apparatus in the lanthorn of the lighthouse, may discover, either derangements of the whole, or of parts. From the expressions of the makers, M. Sautter & Mr. Chance, I understand that these can be rectified in apparatus already erected; and can be prevented in apparatus to be made hereafter:- so that the test proposed makes no undue claim on the manufacturer.

The Royal Commission, when at Whitby, pointed out certain deficiencies in the illumination of the sea; and a waste upon the sky of a portion of light which being indicated by their mode of day examination. The lower reflectors, especially, appeared inoperative, not merely because only a little light could in any case fall upon them, but because if a good lamp had been in the center the adjustment of the reflectors was out[.] A like condition of matters was indicated when at the North Foreland.

Of the two lights at Whitby within 258 yards of each other, the North light lamp is much better than that at the South light; it gives a higher flame and burns more oil, and the appearance at Sea corresponds to this difference. But besides that there are differences in adjustment. An excursion to sea at night on the 9th was made, and at a given signal the whole of the lenticular band of the North light was covered up only the reflectors being left, to compare with the whole of the South light as a standard. In this state the North light was nearly equal to the South light in brilliancy, & in certain positions of the ship was quite equal to it. The light was thus covered up and uncovered again, twice, and the observations were made at distances of four and seven miles. They showed that the upper reflectors at this house were well adjusted to cast the light upon the sea. Here, therefore at Whitby, it appears to me it would be best to make any proposed changes; for they could be carried out at the South lighthouse by Mr. Chance, the maker of the apparatus, who was present and who understands every point in the matter, and the North light could be left as a standard by which to estimate the improvement gained.

I am persuaded that the condition and character of the lamp has a most important influence over the results that have been & are to be acquired. Some persons call a flame 3½ inches high which I and others would consider as only 2½ or 2 inches in the effectual part. Some persons count from the top of the burner to the top of the tongues of flame; whereas the bright luminous part of the flame often begins ⅝ of an inch above the burner, and as a body ceases, it may be, an inch or even two inches beneath the top of the longest smokeless tongues. The difference in the consumption of oil at the different lighthouses shows the great diversity existing amongst the flames of different lamps; to which, if one common standard of adjustment in respect of the optic apparatus be applied, it must as often be wrong as right. It appears to me that the first thing is to have an excellent & constant lamp; and that all lighthouses of the same order should have a lamp of the same quality;- that the glass chimney employed should have a gradually rounded shoulder & not a sharp square one which greatly disturbs the direction of the light;- that when the best form has been attained it should be adhered to;- & that the glass & iron chimney together should be continuous for 6 feet. Then the practicable constant size and condition of the flame of such a lamp should be determined, and a general gauge & measure of the bright part of it & its position in relation to the burner, be supplied to each lighthouse (which could easily be done by an outline drawing on open wire gauze) that the keeper may report whenever the lamp falls short of its required duty. Having such a lamp, it should be ascertained whether the foci for the adjustment of the optical apparatus at present adopted, are the best for it, as the French Authorities believe, or whether they could be advantageously altered; and then apparatus constructed in future should be made in conformity thereto, and finally tested in their place by the Royal Commission process.

If Mr. Chance were authorized to procure such a lamp for the South Whitby lighthouse (ie a lamp which being excellent could be easily & certainly repeated) and were then to adjust the Optic apparatus to it, the result could easily be tested by a comparison with the unaltered North light;- but it would be desirable to be able to ascertain separately, as far as may be, the effect due to the improved lamp, and that due to the re-adjustment of the glass.

With respect to the North Foreland lighthouse, M. Sautter the manufacturer met the Royal Commission & the Trinity Board there, and heard & saw all that passed. I understood him to admit freely the principle advocated by the R. Commission, but to assert that the apparatus was in perfect adjustment for a proper lamp. Setting up the gauges at the burner, according to the focal planes assumed & adopted in France, the apparatus, with the exception of one or two prisms certainly was in beautiful adjustment to them. For my own part, I am of opinion that in relation to the generality of lamps as I have seen them burning, the foci at least of the lenticular part, are taken too high. The chief focus at the North Foreland has been placed 28 millimetres above the burners. As I saw the lamp burning on the Evening of Wednesday the 8th instant and the keeper said it was in its right & usual state, the chief plane of light was not above 22 millimetres above the burner; and if account be taken of the dip of the sea horizon, which is here about 14'.2, the burner ought to be raised on that account 4 millimetres more, making the distance below the focal plane 18 millimetres only instead of 28. If however the lamp were raised enough to correct this difference that would greatly throw out of adjustment both the upper & lower reflectors.

M. Sautter is of opinion that the apparatus requires no adjustment but is correct for a proper overflow lamp. He has full confidence in the French authorized foci. He is however ready to raise the lamp and to readjust the reflecting prisms to any degree the Trinity house may require. Though I think that the foci may perhaps be altered with advantage and intend making an investigation of their places when a good standard lamp is employed if the Trinity board desire it, I am not prepared to go hastily in opposition to the conclusions carefully drawn from theory, experiment, and long practical application by the authorities in France and therefore am of opinion that if MM. Sautter & Wilkins place a proper lamp in the lighthouse and leave it & the optical apparatus in that state of adjustment which the former approves of, and will be responsible for, the determination of any further change there may remain until after the effect has been ascertained of the alterations at Whitby the changes at the North Foreland itself and the re-examination of the places of the foci¹⁰[.]

M. Faraday

Royal Institution | 16 August 1860.