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substituted for 30, and yet an effect would be produced 26 times greater than that of the present light, the most perfect of its kind in this country.”
By similar experiments, it was found that the French lens, a compound one, built up of separate pieces, on a principle first suggested by Sir David Brewster, was equal to 10:4 reflectors, taking into account the effect of the additional lenses and reflectors that ought to accompany it; and that the effect of a single reflector with a lime-ball would be equal to 25 times that of the French lens, accompanied by those additional lenses and reflectors.
“Such appear to be the singular and important results,” says Mr Drummond, “ of our late experiments at the Trinity House. Made with every precaution by different individuals with different instruments, and unbiassed by the knowledge of each other's results, I see no reason to doubt their accuracy; and the comparative appearances of the different lights, when exhibited at a distance of ten miles, though not admitting of being reduced to numbers, confirm the striking superiority of this method of illumination.”
The comparative appearances of the lights, as seen at a distance, are described in a letter, addressed to Mr Drummond by Captain Basil Hall. The lights were exhibited from the temporary lighthouse erected at Purfleet, and were observed from the Trinity Wharf, Blackwall, at a distance in a straight line of 104 miles. Drummond being engaged at Purfleet, could not himself judge of the effect. But among the observers were Sir George Cockburn and Mr Barrow from the Admiralty, accompanied by several naval officers of high standing; the Lord-Advocate of Scotland, Sir Thomas Brisbane, Colonel Colby, Captain Beaufort, hydrographer to the Admiralty, and several others eminent for their professional and scientific attainments. Captain Hall assured Mr Drummond that he had endeavoured “to frame his account of what passed in strict conformity with the general sentiments of the party, and neither to exaggerate nor underrate any of the results."
“ 4 ST JAMES' PLACE, June 1, 1830. “ MY DEAR SIR,—You wished me to take particular notice of last night's experiments with the different kinds of lights exhibited at Purfleet, and observed at the Trinity Wharf, Blackwall; but I have little to add to what I told you respecting those on the evening of the 25th instant; indeed, it is not within the compass of language to describe accurately the details of such experiments, for it is by ocular evidence alone that their merits can be understood.
“ Essentially, the experiments of last evening were the same as those of the 25th, and their effects likewise. The degrees of darkness in the evenings, however, were so different, that some particular results were not the same. The moon last night being nine or ten days older, lighted up the clouds so much, that even when the moon herself was hid, there was light enough to overpower any shed upon the spot where we stood by your distant illumination; whereas, on the 25th, when the night was much darker, the light cast from the tem
ery lighthouse at Purfleet, in which your apparatus was fixed, was so great, that a distinct shadow was thrown upon the wall by any object interposed. Not the slightest trace of any such shadow, however, could be perceived when your light was extinguished, and any of the other lights were exposed in its place.
“In like manner, on the evening of the 25th, it was remarked by all the party at the Trinity Wharf, that, in whatever direction your light was turned, an immense coma or tail of rays, similar to that produced by a beam of sun-light in a dusty room, but extending several miles in length, was seen to stream off from the spot where we knew the light to be placed, although, owing to the reflector being turned too much on one side, the light itself was not visible.
“Now, last night there was none of this singular appearance visible; but whether this was caused by the presence of the moonlight, or by the absence of the haze and drizzling rain which fell during the evening of the 25th, I cannot say. I had hoped that the appearance alluded to was to prove a constant accompaniment to your light, in which case it might, perhaps, have been turned to account for the purposes of lighthouses. If in hazy or foggy weather this curious effect of reflected light from the atmosphere be constant, it may help to point out the position of lighthouses, even when the distance of the observer is so great that the curvature of the earth shall render it impossible for him to see the light itself.
“ The following experiments, tried last night, were the same as those of the 25th, and certainly no comparative trials could be more fairly arranged
“ Exp. I. The first light exposed was the single Argand burner, with a reflector. This was quite distinctly seen, and all the party admitted it be a good light. After several minutes, this was put out.
“ Exp. II. The seven Argand burners were next shown, each in its reflector; and this was manifestly superior to the first; but how much so I cannot say-perhaps four times as conspicuous. Both these lights had an obvious tinge of brown or orange.
“Exp. III. The third light which was exposed (on the seven Argands being put out), was that behind the French lens; and I think it was generally admitted by the party present that this light was whiter and more intense than that from the seven Argands, though the size appeared very much the same.
“ Exp. IV. The fourth light was that which you have devised, and which, instead of the clumsy word ‘lime,' ought to bear the name of its discoverer. The Drummond light, then, the instant it was uncovered, elicited a sort of shout of admiration from the whole party, as being something much more brilliant than we had looked for. The light was not only more vivid and conspicuous, but was peculiarly remarkable from its exquisite whiteness. Indeed, there seems no great presumption in comparing its splendour to that of the sun; for I am not sure that the eye would be able to look at a disc of such light, if its diameter were made to subtend half a degree.
“ The next series of experiments was the most interesting and decisive of all. Each of the lights above enumerated, viz., the single Argand burner, the seven Argands, and the French lens, were exposed, one at a time, in company with your light, in order to try their relative brilliancy.
“ First comparative Experiment.—The single Argand burner was first exposed to this comparative ordeal, and nothing could be more pitiable than the figure it cut. Many of the party could not see the Argand light at all ; while others could just detect it 'away in a corner,' as some one described it. It was also of a dusky orange tinge, while your light was of the most intense whiteness.*
“Second comparative Experiment.--The seven Argand burners were now substituted in place of the single light. All the party could now see both lights, but the superiority was not much less obvious. I really cannot affix a proportion either as to size or brilliancy; but I should not hesitate to say that your light was at least six or eight times as conspicuous; while in brilliancy, or purity, or intensity of light (for I know not precisely what word to use to describe the extreme whiteness), the superiority was even more remarkable. All this which I have been describing was expressed, and appeared to be quite as strongly felt, by the rest of the company, to the number, I should suppose, of five-and-twenty or thirty persons, who were all closely on the watch.
“ Third comparative Experiment.-- The next comparative trial was between the French lens and your light. The superiority here was equally undeniable, though the difference in the degree of whiteness was not so remarkable. The French
* “To many the rays from the brighter light appeared, when seen with the naked eye, to extend across and envelope the fainter light, though the perpendicular distance between them was twenty-five yards."
light, however, is so nearly similar to that from the seven Argands, that the comparison of each of them with your light gave nearly the same results, and all equally satisfactory on the score of your discovery.
" Final Experiment.—The flashes with which the experiments concluded were very striking, and might, I think, be turned to great account in rendering lighthouses distinct from one another. The revolutions were not effective, and, as I said before, there was no appearance last night of those enormous comets' tails which swept the horizon on the night of the 25th, to the wonder of all who beheld them : neither could there be detected the slightest trace of any shadow from the light thrown towards us; and I suspect none will ever be seen, when the moon, whether the night be clouded or not, is of so great a magnitude.
“ Such is the best account I can give of what we witnessed ; and I need only add, that there seemed to be amongst the company but one opinion of the immense superiority of your light over all the others brought into comparison with it.-I &c.
BASIL HALL.” The superior brilliancy of the light being thus established, the only point for farther inquiry was the expense of its production. The result of Mr Drummond's inquiries on this point are set forth in his paper, and are unfavourable to his light. “It may, however, in this, as in every similar instance,” he observes, “ be expected that, after a little experience, a considerable reduction in the cost would be effected.”
“ This is a new source of artificial light, differing from every other at present in use, and the materials by which it is produced are among the most abundant products of nature; but never having yet been applied on a great scale to any practical purpose, it has not hitherto been an object to obtain them in a separate state at a small expense. . Meanwhile, however, the case in question may perhaps be regarded as one where expense ought not to be a primary object of consideration. On all ordinary occasions, the preference of one mode of illumi