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placed on a temporary erection near it. From the first point of observation, near the town of Dunbar, the committee were unanimous in pronouncing the great superiority in effect of the new light, which appeared brighter than the old one, in the rates of not less than four or five to one. But upon changing the position of the point of observation to the eastward, the difference became less marked and at a distance (from the first station) of about one and a half mile, was scarcely appreciable, though the new light appears whiter and somewhat better defined. The reason of this prodigious difference of the old light in different azimuths, Mr. Alan Stevenson, the assistant engineer, explained to be, that since the divergence of the reflected beam is produced by placing the lamp a little out of the focus of the mirror, the central pencil of reflected light is much less scattered than the lateral pencils which in an azimuth exactly intermediate between the axes of two adjacent reflectors, is very feeble indeed. The evening of the 26th was neither remarkably clear, nor remarkably the reverse, and the members of the committee were satisfied that a slight increase of haze would have rendered the old light wholly invisible at the town of Dunbar, which appears to be placed (in the present temporary position of the reflecting system) in the line of least illumination.
The number of mirrors in the old system being twenty-four and the divergence given to the reflected light 15", a certain portion of light is thrown all around the horizon, but the intensity in different directions varies (as has been said) prodigiously. The divergence of 15° in a vertical plane also taking place as a necessary consequence of this method of expanding the reflected beam, by unfocusing the lamp, produces a useless and most injurious expenditure of light in directions in which it can never be seen. On the other hand, the dioptric system of hoops, besides the great advantage obtained by substituting refraction for reflection (from the less loss of light) rigorously fulfils the requisite geometrical condition of spreading a plane of light, exhausting the whole available light of the lamp of precisely equal intensity in all directions. This has never been attempted by any combination of reflectors.
The distance of the Isle of May from Dunbar being thirteen miles, the range of one mirror of the old system extending through about 15", would not differ much from a linear distance of three miles in a direction perpendicular to a line joining Dunbar and the May.
The space between maximum and minimum light would therefore be about a mile and a half. The space walked over must have nearly included a complete series of the variations of the light. Assuming the average light of the mirrors at half that of the dioptric light (which certainly does not seem too much) we have a superiority of two to one in favor of the latter at a distance of thirteen miles. These are to be considered, however, but as rude occular estimations.
But it is not to be inferred that the ratios of the intensities of the two systems will remain the same at all distances. Were the refracted light in one plane (instead of having a virtual divergence of 6°) and were the reflected light comprised within a cone uniformly illuminated (which is not the case) the light from the first would vary inversely as the distance, from the second inversely as the square of the sum of the distance and a constant which depends upon the divergence; but neither of these conditions being fulfilled, the reflective illuminations must vary with the distance according to two other distinct laws, each much more complicated. It is clear, there
fore, that the comparison made at one given distance will not apply to any other given distance. But still the effect of distance in modifying intensity, may be much more accurately estimated in the case of the prismatic than of the reflecting arrangement.
The following conclusions seem to be warranted:-
1. That at a distance of thirteen miles, the mean effect of the new light is very much superior to the mean effect of the old light (perhaps in the ratio of two to one.)
2. That at all distances the new light has a prodigious superiority to the old, from the equality of its effects, in all azimuths.
3. That the new light fulfils rigorously the conditions required for the distribution of light to the greatest advantage.
4. That at distances much exceeding thirteen miles, the new light must still be a very effective one, though to what extent the committee have not observed. The light is understood to be still a good one, when seen from Edinburgh, at a distance of about thirty miles.
GREENOCK, V. P. R. S., Edinburgh.
JAMES D. FORBES, Reporter.
APPENDIX G-No. 1.
Extracts from the reports of the engineer to the Scotch lights commissioners, dated October 27 and November 25, 1835, on the new dioptric light of Inchkeith, (the first lens light introduced into Scotland.)
"On the evening of the first of October a new light, on the dioptric principle of Fresnel, was exhibited at Inchkeith, in place of the reflected light which was finally discontinued on the night of the 30th of September. Since that time repeated observations have been made upon the new light from different distances and elevations, under the varying circumstances of clear and foggy weather; and, before the extinction of the former light, numerous opportunities occurred of comparing the relative powers of the two lights, which were shown in contrast at the same hour every night for some weeks. Before proceeding, however, to notice the results of these observations, I shall briefly describe the apparatus itself, which belongs to the second order of the French system.
"The lamp, which has three concentric wicks, is in every particular the same as that employed in the lights of the second order on the French coast. The light itself exhibits the same general characteristics as the reflected light, being distinguished by flashes recurring once a minute. These flashes are caused by the revolution of a rotary frame which carries the lenses, and the mirrors being arranged in polygons around the axis of rotation and attached to a fixed frame, produce a constant smaller light during the intervals between the flashes, and serve, at the same time, to prolong their duration.
"I need scarcely inform the committee that the flash of the lens in this
new apparatus is far superior, both in brilliancy and size, to that produced by the reflector on each face of the old apparatus, as many of the members, and particularly the convener, on several occasions witnessed the effects of the contrasted lights. But I cannot omit an observation made by Mr. Claud Russell, the accountant of the board, who, in the course of watching the two lights from the Dean's bridge, which is about six and a half miles distant from Inchkeith, saw the flash of the lens light recur several times without the reflected light being visible. This is a very important fact and clearly shows that certain degrees of haze exist, which, though capable of obscuring a weaker light, may be penetrated by the flash of the lens, Another fact of some interest regarding the relative power of the lights, is their difference of color; and so great was this difference on some occasions during foggy weather, that some persons, who were not aware of the arrangements made for the trial, could scarcely be persuaded that the reflected light was not artificially colored, its reddish tinge, acquired by the absorbing effects of the fog, being strongly contrasted with the white and more intense light of the lens. That the new light is more powerful than that which has lately been discontinued, is still farther proved by the fact that the relative maximum intensities of the two lights have been ascertained from repeated comparisons, before the exhibition of the light, by the method of shadows, and are represented by the numbers 400 and 1000, in which unity expresses the effect of a single unassisted Argand flame three-fourths of an inch in diameter. It thus appears that the new dioptric light is about two and a half times more powerful than the catoptric light.
Mr. Alan Stevenson, the indefatigable and distinguished engineer to the Scotch lights commissioners, says on page 4 of his report on the Isle of May light, October 8, 1836:
"The first opportunity I had of observing the reflected and refracted lights at the Isle of May, in contrast, was on the night of the 23d ultimo, at Crail, which is distant about six miles from the light-house. At this place the difference both in regard to the color and size of the two lights was very remarkable, and decidedly in favor of the dioptric light, which appeared considerably larger and whiter than its rival. In walking along the coast I reached a point where the penumbra of the two conjoined reflectors came into view, and the gradual decay of the reflected light to its weakest state rendered the superiority of the new light, which maintained its brilliancy unabated, still more remarkable. Satisfactory as these results may appear, I have yet, by my own observations and those of others, obtained still more unequivocal proofs of the peculiar fitness of the dioptric instruments in fixed lights. On the night of the 24th ultimo I visited Dunbar, which is about thirteen miles from the Isle of May, and observed the contrasted lights from that place in the company of two friends, who took much interest in the success of the experiment.
"The night was hazy, and before the hour appointed for illuminating the new apparatus no light could be discovered; but the time had no sooner arrived than a strong and steady light broke into view, and continued
during the prescribed period with unabated brightness, while the old light could only be observed during fleeting intervals of time, and even then was scarcely visible, and certainly could not have been discovered but by those acquainted with its position in relation to the new light. Immediately before the extinction of the dioptric light I carefully took the bearing of the old light, so as to be able afterwards to find its exact locality; and when the new light was extinguished I found that, even with the assistance of the bearing, I had the greatest difficulty in ascertaining by actual vision the existence of a light at all. In these remarks the friends who accompanied me fully concurred; and one of them, who had an opportunity of repeating these observations at Dunbar, on the night of the 30th ultimo, detailed the result in a letter to me, from which I beg leave to make an extract, fully corroborative of what I have stated above. After alluding to the state of the atmosphere, which was very foggy over the sea, he says: I went to the pier-head, where to my great satisfaction I at once discovered the light. It was much redder than when we saw it last Saturday night, owing, I have no doubt, to the density of the fog; but still it was perfectly visible. Having thus ascertained the exact position of the light, I once more attempted to catch the old one; but though I once or twice thought I discovered something like a star looming in the distance, I am not prepared to aver that I saw it. I would therefore say, that my last night's observation fully justified the conclusion arrived at on Saturday night.'
"Since these observations were made at Dunbar, the engineer of the board has tried, during five successive nights, several interesting experi ments on the lights at the Isle of May, the effect of which he witnessed at various distances on the coast of Fife, and from the deck of the lighthouse tender. The first object of these experiments was to try the effect of the curved mirrors alone; and for this purpose the light-keeper was instructed to have the cylindric hoop of refractors wrapped around with two folds of thick woollen blankets, a single fold having been found insufficient to prevent a large quantity of light from being transmitted. The tender having put to sea, the unassisted light of the prisms and the old catoptric light were seen in contrast, at various distances from five to twelve miles, and it appeared to be the opinion of all on board, that although the reflectors gave a much more powerful light on one tack, the curved mirrors had the advantage on the other tack, according as the axes or penumbra of the parabolic instruments came in the line of vision. At the distance of twelve miles the curved mirrors appeared like a reflector light of an inferior description-a result quite accordant with the value of a hundred and thirty Argand flames, which has been determined as the mean effect of the system of mirAt the lapse of the time mentioned in the instructions given to the light-keeper the refractors were uncovered, and the instantaneous accession of the strong light had a striking effect, which the master of the tender compared to a sudden flash of lightning.' The two lights were then seen in their full power, and the greater whiteness and purity of the refracted light was distinguishable, even where the full effect of a reflector was presented to the eye. The deficiency at the junction of two reflectors rendered this difference much more remarkable; and in his letter to me the engineer says: In sailing around, I find the old light is subject to interstitial defects, and that the refractors are in this respect a great improve
The curved mirrors were then screened, so as to exhibit only the retracting part of the new apparatus along with the reflectors, and the only change which took place was a smail decrease in the volume of the new light, which still retained its greater intensity.
"These observations are strikingly conclusive as to the superiority of the refracted light, the whiteness and purity of which indicates its greater intensity, and justifies the accuracy of the numerical value assigned to these lights in a subsequent part of this report. It is not to be expected that a distant observer can detect the proportions of intensity, which can only be measured by means of a definite standard; and a difference of color is all the indication of a stronger light which ought to be looked for in observations made under such circumstances.
"The important results which I have just detailed leave no room for any further remarks on the superiority of the dioptric instruments to reflectors in fixed lights; and I should scarcely be excusable for saying more upon the subject were it not to add, that while the mean intensity of the light has been increased to considerably more than twice that of the old one, a positive saving has been effected in the expenditure of oil, in the ratio of 17 to 24. Besides, the new light is now entirely free from the defect which is inherent in every fixed light on the catoptric principle-that of being unequally powerful in different directions, according as the axis or the edge of the reflector may be presented to the eye of the observer. The conversion of the fixed lights to the dioptric system would thus appear to be in a measure worthy of general adoption; and there are certain situations where this change would be more especially advantageous, as, for instance, in the case of double lights, in which the equal distribution of light becomes of great importance, in rendering them equally b.illiant at the same distance. The saving which might thereby be effected in the expenditure of oil would also be considerable at double lights; and at the almost inaccessible station of Pentland Skerries, for example, the quantity of oil to be landed would be reduced from fifty to thirty-four casks."
Comparative view of the expenditure of oil in the under-mentioned lights, on the coast of Scotland, if continued on the reflecting or altered to the refracting system.
*This deficiency would have been somewhat less remarkable had there been (as in all the other fixed lights) twenty-four small reflectors instead of twenty-two large ones; but the defect of light at the junction of two reflectors is always too remarkable in any arrangement.