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rations at least, how little advantage foever we may expect from them ourfelves; and yet very poffibly fome obfervations of this fort, and fuch as may be made in a few years, may not only be fufficient to do something, even at

prefent, but alfo to fhew, that much more may be done hereafter, when thefe obfervations fhall become more numerous, and have been continued for a longer period of years.




HAT property of the eye by
which the apparent magnitude of
an object is varied, without any alte-
ration taking place in the angle under
which it is feen, I fhall now call the
magnifying power of the eye. This I
have already explained, and have alfo ad-
vanced that the difference in the apparent
magnitude of the moon in the horizon
and on the meridian is chiefly owing to
this power: and my next object of in-
quiry is, whether it does not likewife
affect telescopic obfervations.


In viewing the moon through a refracting telescope magnifying twenty times, if the object glafs be two inches or upwards in diameter, fhe will apdiftincter than fhe does to the naked eye, but not fo large as might be expected from the power of the inftrument. The reafon, I apprehend, is this, the light being much increafed, the magnifying power of the eye, is thereby diminifhed. But if the fame telefcope be made to magnify 200 times with a field of view ten times lefs than before, the quantity of light entering the eye, in viewing the fame object, will be cco times lefs than with the power of 20. In confequence of which, the magnifying power of the eye will be increased. Hence it appears, that the aftronomer gains an advantage by ufing high magnifying powers in his telefcope: the power of the telescope being affifted by the power of the eye. And a small field of view, on fome occafions, may be preferable to a large one, because the eye thereby will receive less light without diminishing the light of the object.

Thofe parts of the eye which we

have occafion to move almost every time we view a different object, obey our will inftantaneoufly, but fuch parts as we ufe on particular occafions only, are not nearly fo much at our command. The iris expands the moment a ftrong light enters the pupil, and contracts as the light diminishes; we can view an object at the distance of fix inches, and the next moment fee another at the diftance of fix leagues with the utmost diftinctness the eye is capable of, without perceiving that the cryftalline humor altered its diftance with refpect to the retina. But if a. perfon goes into a room where the light is 2 or 3000 times lefs than the light he came out of, it will be a confiderable time before his eyes gain such a form as may be beft fuited to view objects in that fituation. For this reafon, thofe telefcopes which magnify the moft, require the moft time in viewing an object, to fee it in the moft perfect manner.

Perhaps it might be of fome ufe to know, how much the magnifying power of the eye is increafed, when the light is leffened in a given ratio. For example, fuppofe the fun on the meridian gives 2000 times more light than he does in the horizon, and that his diameter appears three times lefs in the former fituation than in the latter; then, I fay, the eye magnifies the object three times, when its light is leffened 2000 times. I think this point may be determined by obfervation on the fixed ftars, made with high magnifying powers. But this I fhall leave to thofe who are provided with proper telescopes, and proceed to exa

mine fome obfervations that are already made.

The very ingenious Mr. Herschel, in his paper on the Parallax of Fixed Stars, fays, "In fettling the distances of double ftars I have occafionally used two ways. Those that are extremely near each other may be estimated by the eye in measures of their own diameters."


The other method is by the micro"As I always make the wires of my micrometer outward tangents to the apparent diameters of the ftars, all the measures must be understood to include both their diameters." And, in another placet, he fays, "That the estimations made with one telefcope cannot be applied to thofe made with another: nor can the estimations made with different powers, though with the fame telescope, be applied to each other. Whatever may be the caufe of the apparent diameters of the ftars, they are certainly not of equal magnitude with the fame powers in different telescopes, nor of proportional magnitude with different powers of the fame telescope. In my inftruments. I have ever found lefs diameter in proportion the higher I was able to go in power."

To account for thefe feeming irregularities it will be neceffary firft to mention, that the apparent diftance between two stars will be increafed in proportion to the magnifying power of

the telescope, when they are viewed in the fame ftrength of light.

That when the power is increased the light is decreafed in the inverfa ratio of the fquare of the power: wherefore the apparent diftance will be increafed in the compound ratio of the powers of the eye and telescope.

And, "that the diameters of the fixed ftars are not proportionally magnified with higher powers as the planets are." From fig. 2 and 5§ ie feems that three times the power about doubles the apparent diameter.

From thefe theorems and obfervations it will be eafy to account for the obfervations of Mr. Herfchel refpecting the apparent magnitudes of double ftars. For the distance between two ftars increafing fafter by magnifying than their magnitudes, the higher the magnifying power is with which they are viewed the lefs will their apparent diameters be when compared with their apparent distance as under.

Hence the apparent magnitude of a ftar cannot appear proportional to the different powers used in the fame telefcope, when its distance from another ftar is ufed in eftimating or calculating that magnitude. Nor will a ftar appear of an equal magnitude with equal powers in different telefcopes, because they may afford different degrees of light which will alter the magnifying power of the eye.






S the attention of the world has been much excited by the powerful effects

are many are defirous

of it provided its advantages were clearly afcertained, I prefume the following defcription of the inftrument and its effects will not be unacceptable to the public. Your's, &c.

THE apparatus confifts of two principal parts, a fountain to contain the oil, and the lamp itself. Of the


former it is unneceffary to speak: the lamp is conftructed as follows. The external part confifts of an upright metallic

Philof. Tranf. Vol. LXXI. p: 492,

* Philof. Trapf. Vol. LXXI. p. 99. + Ibid. p. 102.
$ Ibid. Vol. LXXII, tab. 4. p. 410.

tallic tube one inch and fix-tenths in diameter, and three inches and a half in length, open at both ends. Within and concentric to this is fixed another tube of about one inch in diameter, and nearly of equal length; the space between thefe two tubes being left clear for the paffage of the air. The interior tube is clofed at the bottom, and contains another similar tube a little more than half an inch in diameter. This third tube is foldered to the bottom of the fecond. It is perforated throughout fo as to admit a current of air to pafs through it, and the space between this tube and that which invirons it contains the oil. An ingeious apparatus, containing a piece of cotton cloth whofe longitudinal threads are much the thickeft, is adapted nearly to fill the fpace into which the oil flows. It is fo contrived that the wick may be raised or depreffed at pleafure. When the wick is confiderably raised it is feen of a tubular form, and by the fituation of the tubes already defcribed is acceffible to the air, both by means of the central perforation and the space between the exterior and fecond tube. When the wick is lighted, the flame is confequently in the form of a hollow cylinder, and is exceedingly brilliant. It is rendered fomewhat more bright, and perfectly steady, by adapting a glafs chimney whofe dimenfions are nearly the fame with that of the exterior tube firft defcribed.

I hope this short defcription will be fufficient to convey an adequate idea of the inftrument, and fhall therefore proceed to mention its effects. If the central hole be ftopped, the flame changes from a cylindrical to a pyramidical form, becomes much lefs bright, and emits a confiderable quantity of fmoke. If the whole aperture be entirely or nearly ftopped the combuftion becomes ftill more imperfect. The accefs of air to the external and internal furfaces of the flame is of fo much importance, that a fenfible difference is perceived when the hand or any other flat fubftance is held even at the distance of an inch from the lower aperture. There is a certain length of

wick at which the effect of the lamp is the beft. If the wick be too much depreffed, the flame, though white and brilliant, is fhort; if it be raised, the flame becomes longer, and confequently the light more intense and vivid. A greater increase of the length, increafes the quantity of the light, but at the fame time the upper part of the flame assumes a brown hue, and smoke is emitted.

The lamp was filled with oil and weighed, it was then lighted and suffered to burn fo as to produce the greateft quantity of light without fmoke. After burning one hour and fifty-two minutes it was extinguished, and found to have loft 589 grains of its weight. Now a pint of the oil weighs 6520 grains, and cofts fixpence three farthings in retail: the lamp therefore confumes oil to the value of one penny in three hours. It remains to be shewn at what rate per hour the fame quantity of light might be obtained from the tallow candles commonly used in families.

The candle called a middling fix, weighing upon an average the fixth part of a pound avoirdupois, is 10 inches long, and 2 inches and in circumference. I have chofen to make my comparison with this candle as being, I imagine, most commonly used. It is to be understood that the lamp gave its maximum of light without fmoke.

The best method of comparing two lights with each other, that I know of, is this: Place the greater light at a confiderable diftance from a white paper, the lefs light may be moved nearer or farther from the paper, accordingly as the experiment requires. If now an angular body, as the most convenient figure, be held before the paper it will project two fhadows, these two fhadows can coincide only in part, and their angular extremities will in all pofitions but one be at fome diftance from each other: the fhadows being made to coincide in a certain part of their magnitude, they will be bordered with a lighter fhadow, occafioned by the exclufion of the light from each of the two luminous bodies refpectively.

refpectively. Thefe lighter fhadows in fact are spaces of the white paper illuminated by the different luminous bodies, and may with the greatest ease be compared together, becaufe at a certain point they actually touch one another. If the fpace illuminated by the lefs light appear brighteft, that light is to be removed farther off; and, on the contrary, if it be the moft obfcure, that light must be brought nearer the paper. A confiderable degree of precifion may be obtained by this method of judging of lights, and by this method the following comparifons were made.

The candle was fuffered to burn till it wanted fnuffing fo much, that large lumps of coaly matter were formed on the upper part of the wick. The candle then at the diftance of 24 inches gave a light equal to that of the lamp at the distance of 129 inches from this experiment it is deduced that the light of the lamp was equal to about 28 candles. The candle was then fnuffed, and it became neceffary to remove it to the diftance of 67 inches, before its light was fo much diminished as to equal that of the lamp at the beforementioned distance of 129 inches. From this experiment it is deduced that the light of the lamp was equal to not quite four candles fresh fnuffed. Another trial with the lamp at the diftance of 131 inches and a half, and another candle of the fame fize at the diftance of 55 inches gave the lights equal. The candle was fuffered to burn for fome time, but did not feem to want fouffing, yet the light of the lamp then appeared to be the ftronger. The candle when newly fnuffed, the distances remaining the fame, appeared rather to have the advantage of the lamp. These numbers give 5 candles for the light of the lamp, and I imagine the lamp to be rather better than this upon an average, because candles are fuffered to go a much longer time without fnuffing, and therefore in general give lefs light than was exhibited in thefe trials. Another trial with the lamp raised fo as to fmoke a little, and the candle wanting fnuffing, though the form of the wick LOND. MAG. May 1785.

had not as yet began to change, gave the proportion of the lamp to the candle as about 8 to 1. We may, therefore, I prefume, take 6 middling fixes of tallow, candles as an equivalent in light to the lamp. I tried the lamp againft 4 candles lighted up together, placed on a diftant table with the lamp, I retired till I could juft difcern the letters of a printed book by the light of the candles, the lamp being covered. I then directed my affiftant to intercept the light of the candles and fuffer the lamp to fhine on the book; the lamp was the brighteft. It seemed by trials of this kind to be rather better than five candles; but I was not at that time aware of the difference of the light of tallow candles, accordingly as they have been more or lefs recently fnuffed, and as this method does not appear capable of that degree of exactnefs and facility the other poffeffes, I did not purfue it.

- From these trials it is evident that where light beyond a certain quantity is wanted, at a given place, thefe lamps muft be highly advantageous; for the tallow candle being of fix in a pound, and burning not quite seven hours, the lamp is equivalent to a pound of these candles lighted up for feven hours. Now, the expence of the lamp for feven hours is less than two pence halfpenny, and that of the candles eight pence; and if the proportion between wax and tallow candles be attended to, it will be feen that the advantages of this lamp for illuminating a theatre are very great. The wax candles in Covent-Garden theatre are about eighty in number in the fconces, and by eftimation may be worth about 21. fterling. An equal quantity of light would be afforded by fourteen of the patent lamps: for the candles ufed at the theatre do not give quite fo much light as a tallow candle of fix in a pound. The expence of the fourteen lamps for five hours will not exceed two fhillings, according to the foregoing deduction.

Mr. Argand is certainly entitled to all the honour which his talents for philofophical combination have gained; and in the prefent inftance, his claim

U u


as an inventor ought not to be difputed, though it fhould appear that the principle of his lamp was known and even applied to ufe long ago. Every one is acquainted with the obfervation of Dr. Franklin, concerning the increase of light produced by joining the flames of two candles: and double candles have actually been made for, and used by fhoemakers, from time immemorial. The lamp of many wicks ranged in a right line, and used by watchmakers, gives a very great light for the fame reafon, namely becaufe the flame being of no confiderable thickness has accefs of air throughout, and the combuftion is perfectly maintained. Whereas in a thick flame the white heat or perfect ignition extends only to a certain diftance from the exterior furface. This is exemplified in a ftriking manner in thofe large flames which iffue from the chimnies of furnaces. These are luminous only to a certain diftance inwards, and the interior part confifts of vapour, hot indeed, but not on fire, fo that if paper be held in the centre of the flame by means of an iron tube paffed through the exterior burning part, the paper will not be fet on fire. Mr. Argand has propofed the converting a right lined wick into a circular one: whether this be an advantage or no, exeept fo far as concerns the convenience of having a longer range of conjoined flames within a lefs fpace I was defirous of afcertaining. The refult of trials are these.


I took one of Mr. Argand's wicks, which when cut open longitudinally will form a line at the extremity propofed to be lighted, meafuring about two inches and fix-tenths. This wick was placed in a brafs trough, fo that the upper edge of the wick was held perpendicular by the trait edge of the trough into which oil was put. The wick was then lighted, and it was eafy to raise or lower it above the metallic edge at pleafure, because it adhered by means of the oil to the fide of the brafs veffel. I thus obtained a flame in a right line equal in length to the periphery of Argand's flame, and as is the cafe

in that lamp, I found it easy to lengthen or fhorten the flame, to caufe it to fmoke or burn clear as has been before mentioned. The lamp and this right lined flame were placed near each other, and at the fame height, the glafs chimney being taken off the former: the flames of both were adjusted fo as to emit a fmall quantity of smoke, and their lights tried. The experiment being made by means of the fhadows, as before described, their lights proved exactly the fame: but to the eye, looking at both lamps together, the intenfity of Argand's flame appeared confiderably the greatest; that is to fay, it dazzled more and left a ftronger impreffion when the organ of fight was directed to fome other object.

Before I made this experiment I had fome expectation that the long flame would be preferable to the circular one, because I fuppofed the interior surface of the circular flame, could not throw out fo much light as it would have done if it had been developed and expofed. I was even inclined to imagine that the greater part of the light of Argand's lamp is furnished by the external furface of the flame. But the equality of the lights in the circular and the right-lined flames, fhews that this opinion was ill founded, and that flame is in a very high degree tranfparent. I therefore directed my attention to the fhadow of a lighted candle, and obferved, that when the candle does not fmoke, the fhadow is nearly the fame as if the candle were lighted; that is to fay, as if there was no flame. But, if a piece of glass be held up in the fame light, it will give a fhadow fufficiently fenfible; it therefore intercepts more of the light than flame does. This obfervation accounts for the fuperior brightness or dazzling of Argand's lamp. For the light which falls on a given portion of the retina of the eye from Argand's lamp is much more denfe, because it confifts not only of the light from the anterior but likewife from the pofterior part of the flame. My ideas on this fubject were farther confirmed by an experiment I made with the two lamps; I placed the right-lined flame in fuch a direction



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