Faraday report to Trinity House   23 July 1840

Suggestions, &c.

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To the Master, Deputy Master¹, and Elder Brethren of the Trinity House.

My Lords and Gentlemen,

1. The principle of directing light by refraction for lighthouse purposes is almost perfect, but the practise is necessarily very far from being so; in consequence, partly of defects in the glass employed, and partly of defects in the workmanship and arrangement of the different portions of glass in the combined refracting apparatus.

2. Having been called upon to consider the glass with respect to it’s properties of uniformity, colour, transparency, &c, I have made arrangements to investigate practically the effect of these circumstances. The preparations will require time and occasion delay. In the meanwhile I beg respectfully, to offer the following suggestions or remarks for the consideration of the Board.

3. The application of refracting apparatus by the Trinity House is a matter altogether practical in its nature. Hence it would seem proper that the refractors used should in every case undergo an examination, to ascertain whether they are fit for their purpose and in what degree they are more or less fit:- just as guns or [blank in MS] or chain cables &c. are proved and tested before being applied to their respective uses.

4. The examination should not be one dependant upon strict rule or principle but altogether practical. I mean, that it should not enter into an enquiry as to the true refracting power of the glass, the precise angle of the parts, or their respective position; but should be founded on the real final course of the refracted rays. It is, no doubt, essential for the best result that the points first mentioned should be strictly attended to in the manufactory, but that is not necessary or expedient in the check examination to which the manufactured article should be subjected. The instruments should be constructed on accurate principles and by strict rules; but they should be examined by their effects.

5. The examination wanted, is one, which, not needing abstruse or peculiar knowledge, shall be such as any person with a little experience, might make. It should be sure in its results and yet practicable, both by master and workman; by seller and buyer.

6. Perfection cannot be expected, and as every refracting instrument must necessarily come far short of it, so it is only that degree of perfection which, experience has shown, can be practically attained which ought to be required. This degree would probably be best ascertained by having, as in other similar cases, a standard. Thus calling that a frame which is included within a single metallic boundary as now received by Mr. Wilkins, if the different frames of which the dioptric apparatus is to be constructed were to be examined as they were received, it would be well to agree upon one to be preserved, as a standard of quality, and then compare all other and future ones with it, until a new standard is chosen.

7. When a frame has been examined the general results should be recorded. For this purpose it would be well that the frames should be numbered from 1 onwards as they are manufactured, or else as received. This should be done permanently upon the metal, so that the number, being visible may be referred to at any future time. A record should also be made and preserved of the lanthorns or lights into which the frames enter as parts; and it would be well that the frame book should contain a brief description of each frame, it’s size, focal length, kind of glass, date of manufacture, direction of refraction, and other important points.

8. Such an examination as that proposed, beside serving to pass the frames, would also answer other useful purposes. It would give the power of selection for particular lanthorns; for it might often happen that a frame not thought sufficiently good for one situation might do for another. It would give the power, also, of determining with advantage the position of the frame in the lanthorn; for a certain frame may be more fitly placed in one part of a lanthorn than another according to the circumstances of the sea view in different directions from the light. Or it may even be important to determine which end of a frame is placed uppermost. To understand the latter case, it will be enough to point out that the central horizontal part of any frame refracting only in a vertical plane is the most important, the part above this is the next in importance, and the part below, the last. The reason is that the burner because of it’s position and size cuts off part of the light from the lower portions of the refracting arrangements, whilst the corresponding rays are not cut off from the corresponding parts of the lens or frame above. Hence if one end of a frame is found to be better than the other, that end should be placed uppermost in the lanthorn.

9. The frames having been examined individually, should again be examined as to their joint arrangement in the lanthorn or refracting system of a given lighthouse. To meet this examination they should be so built together as to give a perfectly horizontal medium beam in every direction. For, if, from the height of the lighthouse, it should be desired to depress the beam a certain degree so that it may fall on the sea horizon, I think that it would still be desirable for the securing of accurate workmanship, and the application of the test, to make the lamp yield a horizontal beam in the workshop, and afterwards obtain the depression of the ray by elevating the lamp the quantity required.

10. If a case occurred in which from the position of the lighthouse, it were desired to give different degrees of depression to the ray, in different directions, then, it would be better to have that determined before hand; wrought up to in the manufactory; and allowed for in the examination. Such a determination would not affect the individual frames, but only their arrangement in the lanthorn.

Proposed mode of examination.

11. My proposition is to examine each frame as if it were a single well wrought piece of glass; and to observe how nearly it approaches to such a piece of glass.

12. Let us first consider the case of a frame intended to refract in a vertical direction only i.e. as in a fixed dioptric light, intended to gather rays passing upwards and downwards from a centre light and cast them in a horizontal direction.

13. Suppose such a frame to consist of a single piece of glass, without steps, (instead of horizontal bands or prisms) well formed, placed vertically, and a small object as a half inch round white ball in it’s focus; and then suppose the eye of the observer placed on the other side of the frame, three or four feet from it, and regarding the object through the middle part of the glass.

14. If the observer were to retire from the frame and still regard the object, it would gradually and regularly grow longer, upwards and downwards, but not wider: approximation would as regularly diminish the apparent length of the object.

15. If, keeping the eye at the same distance from the glass, the head were moved to the right or left, the apparent place of the object would not seem to change in respect to it’s real place, nor would it change in apparent width or height or any other quality or character.

16. If, keeping the eye at the same distance from the glass, the head were lowered, then the apparent place of the object would change; and it would seem to travel regularly from the middle to the lower edge of the glass, as the eye became level in succession with the different parts. As the eye returned to its first position the object would seem to return on it’s course to it’s first place. Whilst the apparent place of the object is thus made to pass towards the lower part of the glass, it becomes lengthened in a vertical direction, it’s upper and lower edges become gradually and regularly coloured. The former or that nearest the middle of the glass being red, orange &c, and the later violet blue &c. If a square object be used, the vertical sides do not change in colour.

17. Similar effects occur if by carrying the head upwards, the apparent place of the object be made to pass in that direction.

18. If the frame instead of containing a single piece of glass, contained (as is the case in practice) many horizontal bands or prisms, all wrought and arranged perfectly so as to produce, as nearly as might be, the effect of a single piece, then, the effects would be the same in kind and occur in the same order and with the same regularity; but there would be an additional effect of the following nature. As, by the motion of the eye, the apparent place of the object was made to move upwards or downwards across the junctions of the different prisms, there would appear at each junction a narrow band, in which the object would not be seen, but across which it would go invisibly; this is an effect due to the different thickness of glass through which the rays have to pass above and below these junctions. The width of these dark bands, so to call them, is largest at the junctions nearest the upper and lower edges of the frame, but they do not interfere with the regularity of the other appearances.

19. The appearances described are very simple in their nature and easily observed and comprehended; they offer also a very sufficient test of the quality of the frames with regard to workmanship and one that can be comprehended and applied by any person after half an hour’s practice.

20. If, instead of being only three or four feet from the glass, the observer is at a greater distance, as ten feet, then the appearances are the same in kind but the test is proportionately more delicate, and any irregularities in the final direction of the rays are more quickly detected. At greater distances the irregularities appear still greater.

21. As a focal object, quite sufficient for the practical application of the frames, I would recommend a flat white circle or square, half an inch wide, fixed upon a black paper or card of three or four inches width. The white object should be adjusted in a vertical position at the focus, and, if a square, with it’s sides vertical and horizontal. It should be steadily and well illuminated, for which if day light is not convenient, a shaded lamp with a bull’s eye or a reflector may be used. The large lamp burner should be removed in the examination for it’s anterior part shades the place of the focus object from the lower part of the refracting frame. A flat surface is better, as an object, than a round white ball, because the latter is not equally illuminated in every direction as seen from the different parts of the frame or lens.

Let us now suppose the frame A placed vertically (13.); the illuminated object B adjusted in the focus (21.); and the eye of the observer at C, ten feet distant from the frame, and let us suppose an examination to take place.

If, as the eye ascends from C to D (16. 17.) the object B appears in succession in the different bands 1.2.3.4.&c (18.) it will shew that the rays refracted by each of these bands pass in their right directions or nearly so, with respect to each other, within the given distance of ten feet. If, then, on increasing the distance of the eye to twenty feet the same effect be observed, it will shew the same thing for that distance; and this degree of test is as great as, or perhaps greater than, any compound lens or frame, intended merely for lighthouse purposes should be subjected to.

23. But on carrying the eye from C. to D the object may not appear successively in the several bands or prisms in their proper order, but irregularly as thus; first in 1, then 2, then in 4, then in 3 and 5 together, next in 7 and lastly in 6. This would shew that the prisms were not refracting their medium rays in horizontal and parallel directions, but sending some up and some down, in a manner quite at variance with the intention of a dioptric light. This will be seen in a moment by the diagram, if we suppose the variations to occur as the eye descends from C to E, by the direction of the lines of sight. The ray from 1 is, thus far, in it’s right place; the ray from 2 may be in its right place; the ray from 3 can hardly be right, for it and the ray from 4 evidently converge to an intermediate point F, and after that go on diverging to any distance; so also the rays from 3 and 5 converge to the place of the eye at G and after that must diverge again. Of these three rays, therefore, only one can by possibility be horizontal; the two others must either pass above or below the horizon. Of the rays from 6 and 7, one or both must be wrong.*

24. A closer judgment of the quality of the frame, and whether the rays from the irregularly acting prisms were thrown above or below the horizontal line, might be made by observing the distance through which the eye must move to see the figure of the focal object in the given prisms; for according as that distance is much or little; or more or less than the distance between the prisms whose images are observed, may a judgment be formed of the extent and direction of existing errors. For instance, suppose the focal object appeared in the right order, as the eye travelled from C to D, but that before it had disappeared from prism 4, it appeared in prism 5, and that it disappeared from prism 5, a little while before it appeared in prism 6; there being a short period or small space in which no object was visible; then it would be evident that the ray from prism 5, converged to that from 4, and diverged from that sent by 6, so that one at least of these rays must be in the wrong direction. But I refrain from remarking further on this kind of observation: for if a frame act pretty well at ten feet distance, and it be required to apply a stronger test to it, it will be better for an ordinary observer to make the distance twenty feet and then go over the same observation as before, than use the method just referred to.

25. Whilst the examiner is observing how the focal object appears in the prisms successively, it may be that, its image may move across a given prism in one direction as the eye moves in the other; thus as the eye rises from C to D, the object, on appearing in prism 4, may seem to enter at the top edge and disappear at the bottom edge. This effect indicates, not that the prism is wrongly adjusted as respects the other prisms, but that it has not the right angles; that it’s focus is not properly adjusted; that it ought to be nearer the light. If the object appears in the whole width of the prism at once, it indicates an effect of the same kind but in a smaller degree. If the object enter and disappear rightly as to order but continues in the prism for too long a time and, whilst it is present either in the one above, or in that below, then the angles are also wrong but now the glass is too near the object and the prism requires removing further from the focus to throw parallel rays.

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26. This part of the examination may now be considered as finished; but the central part of the frame or that near the middle horizontal plane should undergo a further examination, by keeping the eye at the same altitude, but moving the head right and left. This makes the figure of the object appear in different parts of the frame on the same horizontal line. Such a change of place should cause no change in the length or appearance of the object. If a change do occur, then the single piece of glass in which it is observed is worked irregularly, and it’s different parts are inconsistent with each other in their action.

In concluding this examination or rather the instructions for it, it is to be remembered as a kind of general rule, that, for the object to appear too large, is a fault as great as for it to appear too small. If on moving the head to the right or left (15.) the object appear enlarged in any particular part of the glass, then, above or below that place of enlargement will be found another place where the image of the object is either too small or will disappear altogether; or if in any position it seem large from its appearing in two or three prisms at once, then there are sure to be corresponding positions in which no object will be seen, and to which of course no light would be directed if a lamp were put in the place of the test object[.]

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28. Another simple, useful, practical, and proper mode of examining a frame is to place it upright, fix a proper lamp in it’s focus, and then observe the general refracted beam by it’s effects on a white screen placed at different distances from the glass. A frame containing a single perfectly wrought piece of glass would throw a nearly uniform light on to a screen placed close to it, but coloured a little on its upper and lower edges; the screen being withdrawn gradually, the mass of light on it would still appear nearly the same in vertical dimensions, but more and more diffuse at the upper and lower edges, from the inevitable effect of the refractive and dispersive power.

29. If the frame consisted of a compound glass, the parts of which were perfectly wrought and adjusted, the appearance of the light on the screen when near to the glass would be as before, but with dark horizontal bands, corresponding to the junctions of the prisms, running across it. As the screen was removed from the frame these would weaken and widen, and at last nearly disappear, giving at 20. 30. or 40 feet a result sensibly the same as that with the single glass (28.).

30. But if a frame be thus examined, which, by the former mode of examination proved to be irregular in it’s action, horizontal bands more or less extensive in size, will appear, and as the screen is placed at different distances, these will change, and point out exactly the course of the refracted rays. Thus some of the rays will be seen to converge, giving bright bands at certain distances edged by dark bands; and if the screen be moved still further, these converging rays will be observed to diverge, and so where a bright band had appeared, a dark one will now come in; and indeed all sorts of variations may with a bad frame be observed and referred to their true courses in error of workmanship or construction.

31. As a final observation, the beam thrown upon the screen should be examined at 40 or 50 feet distance and there the brightest part of it should have a width not much more than the vertical dimension of the frame; and though it cannot be expected that the part above or below should have no diffuse light upon it, yet the quantity of that light should not be great. The allowable proportion of this scattered light would be best expressed by reference to a standard frame.

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32. The examination has thus far been considered as applied to a frame or glass refracting in one place only, namely the vertical plane; but if the instrument to be examined be a lens, then the same principles apply exactly, with simply this variation, that the effects which have thus far been sought for from the horizontal line upwards and downwards only, have now to be observed from the centre of the lens outwards in all directions. The observations then become as easy, practical, and exact as in the case already fully described.

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33. The frames should not only be examined by themselves, but when finally combined for the lighthouse <(9)>. The principal points here, would be to determine whether the frames were placed at the true focal distance from the lamp so as to give the best possible parallel beam; in reference to which point the frames on opposite sides of the lamp ought especially to be compared in their action: and whether each frame gives a truly horizontal beam. These points could be easily ascertained at Mr. Wilkins’ manufactory provided the lanthorns, i.e. the whole arrangement of glasses and mirrors were constructed on a revolving platform. The focal distance of the frames could then be observed either by the use of the screen (28.) or by the horizontality of the beams from any given part of the apparatus;- and the horizontality of the mean ray could be ascertained in the same manner. I find the tube which I sent in to the Board quite sufficient in my hands to prove and determine the horizontality of the beam of light without any reference to the workmanship and adjustment of the illuminating apparatus.

34. When the frames are fitted together it is very interesting to compare the two which are contiguous by the light which they throw on to the screen. If the frames are good the action is nearly alike with both, and at a short distance becomes uniform. If they are bad, the effects are most strikingly developed by the contrast afforded; for it rarely happens that the two accord in the direction given to the issuing rays.

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35. I have suggested the examination first of the frames as they are received, and next of the complete conjoined arrangement, as it is intended to be used in any given lighthouse. It would be easy to give similar simple practical instructions for the examination of each separate rib or piece of glass; but I think that is unnecessary under the present circumstances. The points of colour and striae in the glass, I have already mentioned as being yet to be considered (2.) but I may in the mean time say that I believe errors in the angles, workmanship, and arrangement of the apparatus are of far more consequence than any faults of colour and want of uniformity in the glass which are likely to occur.

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36. I would again venture to suggest as a thing most fit and proper that the mirrors when added in a lighthouse to the dioptric arrangement should be examined also; the necessary degree of accuracy being estimated by a given practical standard. Two points would especially require attention in this examination. The first is that the mirrors should have the right curvature and foci for the places which they occupy;- a very slight examination has shewn that at present great confusion occurs in this respect and that mirrors of different foci have their places allotted them entirely by accident. The second is that each mirror should throw its reflected beam in a horizontal direction.

37. I am of opinion that here again, the best practical mode of checking the workmanship and construction is, not by the application of refined rules, but simply by direct observation of the course taken by the ray after its reflection. This can very simply be done according to the practice already laid down, by looking at, and noting the appearance and place of the image of a focal object, using either the unassisted eye, or employing the tube I have already described or some similar instrument for that purpose².

With all willingness to aid in such examinations as have been spoken of,

I have the honor to be | My Lords and Gentlemen, | Your most obedient | humble Servant, | (signed) M. Faraday.

Royal Institution | 23rd July 1840.

* The sight tube which I described in a former paper will tell instantly which of these rays is right and which wrong.