and marked the size and shape of the image into which it was converted. The perfection of this image, or its narrowness in the direction of the length of the spectrum, became a precise and unequivocal test of the fitness for distinct vision which belonged to the light out of which it was formed.

466. By this method of observation he found that a distinct image of the luminous disc could not be obtained either by producing a blue or a green image, and that it was only in the red portion of the spectrum that such an effect was likely to be obtained. In the use of purple glasses it was observed that the middle portion of the red space was absorbed before the two extreme portions, so that instead of one red image there were two quite separate, and tolerably distinct. By increasing, however, the thickness of the plate, the most refrangible red image was absorbed, and the least refrangible one left in a state of the most perfect distinctness. Although he had now determined the part of the spectrum that was best fitted for giving perfect vision, yet the quantity of light extinguished before the insulation of the extreme red ray was affected was so great as to render the determination of little practical utility, excepting in cases where the outline of an object was to be observed. Had it been possible to insulate the most luminous rays of the spectrum as perfectly as the extreme red ones, the advantage would have been of very considerable amount; but he found this quite impracticable. 467. Abandoning, therefore, all hopes of obtaining from colored media any farther improveinent upon the microscope than what had been formerly announced, it occurred to Dr. Brewster that the object which he had in view might be obtained, if he could procure from the combustion of inflammable substances a homogeneous flame for illuminating microscopic objects.

468. It had long been known that a great quantity of homogeneous yellow light was created by placing salt or nitre in the white flame of a candle, or in the blue and white flame of burning alcohol. A light, however, generated in this manner, was more fitted for a casual experiment than for a permanent source of illumination; and, as insalubrious vapors are disengaged during the combustion of these salts, he did not avail himself of this method of obtaining yellow light.

469. After numerous experiments, attended with much trouble and disappointment, he found that almost all bodies in which the combustion was imperfect, such as paper, linen, cotton, &c., gave a light in which the homogeneous yellow rays predominated; that the quantity of yellow light increased with the humidity of these bodies; and that a great portion of the same light was generated when various flames were urged mechanically by a blow-pipe or a pair of bellows. 470. As the yellow rays seemed to be the product of an imperfect combustion, Dr. Brewster conceived that alcohol diluted with water would produce them in greater abundance than when it was in a state of purity; and, upon making the experiment, found it to exceed beyond his most sanguine expectations. The whole

of the flame, with the exception of a small portion of blue light, was a fine homogeneous yellow, which, when analysed by the prism, exhibited faint traces of green and blue, but not a single ray of red or orange light. The green and blue rays, which accompanied the yellow flame had comparatively so little intensity, that they disappeared in the processes of illuminating and magnifying the object under examination; and, even if they had existed in greater abundance, it was quite easy to absorb them at once by the intervention of a plate of the palest yellow glass, and thus render the lamp perfectly monochromatic.

471. From many experiments on the combustion of diluted alcohol he found that the discharge of yellow light depended greatly on the nature of the wick, and on the rapidity with which the fluid was converted into vapor. A piece of sponge, with a number of projecting points, answered the purpose of a wick better than any other substance, and the extrication of the yellow light became more copious by placing a common spirit-lamp below the burner of the other. In order to obtain a very strong light, for occasional purposes, he connected with the top of the burner a frame of wire-gauze, which by moving vertically round a hinge, or by a motion to one side, could be placed in a horizontal position, about half an inch above the wick. As soon as it had become red-hot it was made to descend into contact with the sponge, when it converted the alcohol rapidly into vapor, and produced an abundant discharge of yellow light.

472. If a permanently strong light is required, it is found preferable to dispense entirely with the use of the wick, and to allow the diluted alcohol to descend slowly from the rim into the bottom of a concave dish of platinum, kept very hot by a spirit-lamp placed beneath it. The bottom of the dish is made with a number of projecting eminences, in order that the film of the fluid which rests upon it may be exposed at many points to the action of the heated surface. After the lamp has burned for some time, a portion of unevaporated water, mixed with a small quantity of alcohol, will remain at the bottom of the dish, in a state unfit for combustion. This water may be taken up by a sponge, or it might be prevented from accumulating, by having a fountain of pure alcohol, from which the exhausted strength of the diluted fluid could be renewed.

473. The monochromatic lamp being thus completed, Dr. Brewster lost no time in applying it to the illumination of microscopic objects. The effect which it produced far exceeded his expectations. The images of the most minute vegetable structures were precise and distinct, and the vision in every respect more perfect than it could have been, had all the lenses of the microscope been made completely achromatic by the most skilful artist.

474. Independent of its use in microscopical observations, the monochromatic lamp will find an extensive application in various branches of the arts and sciences. In certain cases of imperfect vision, where a number of colored images

are formed by the separation of the fibres of the crystalline lens, a homogeneous light will improve the vision, by removing the prismatic tints, which obliterate the principal image. In illuminating the wires of transit instruments and micrometers; in graduating the limbs of divided instruments, which is generally done by candlelight; in reading off the same divisions in fixed observations; in forming signals in trigonometrical surveys; in obtaining correct and uniform measures of refractive powers; in measuring the separation of the two pencils in doubly-refracting crystals; in determining the focal lengths of lenses; in observing various optical phenomena, where the light is decomposed; in these, and in general in all delicate works, where correct vision is essential, the employment of a homogeneous flame will be found to confer the most signal benefits.

475. Fig. 10 represents one form of the monochromatic lamp, where A is the reservoir containing the diluted alcohol, which descends by the channel A B C D to the broad wick E, which is generally made of sponge. A frame of wiregauze F moves round a hinge H, so that it can be brought over the flame, and made to descend, when hot, upon the surface of the wick. Excellent wicks may be made with concentric cylinders of thin mica, or of platinum foil.

476. Fig. 11 is another form of the lamp, without a wick, in which the diluted alcohol is burned in a flat platinum or metallic dish M N, which may be made to have a slight spontaneous oscillatory motion, for the purpose of bringing the fluid over the heated projections of the platinum. A common spirit-lamp OP, enclosed in a case, is placed below the platinum-dish MN, in order to produce sufficient heat for throwing off the vapor from the diluted alcohol. A chimney, or a cylinder, of pale yellow glass may be placed round the flame, if it should be thought of any consequence to absorb the small portion of blue light which accompanies the yellow flame.

477. The photometric instrument we are now about to describe is likely to be of the utmost importance in the science of optics, and as Mr. Ritchie has just furnished the Royal Society with an account of the theory on which it is constructed, as well as the delicate minutia essential to its completion, our readers may for a few hillings readily construct a similar apparatus.

478. The instrument consists of two cylinders of planished tin plate, from two to ten or twelve inches in diameter, and from a quarter of an inch to an inch deep. One end of each cylinder is enclosed by a circular plate of the same metal, soldered completely air-tight, the other ends being shut up by circular plates of the finest and thickest plate glass, made perfectly air-tight. Half way between the plates of glass and the ends of the cylinders there is a circular piece of black bibulous paper, for the purpose of absorbing the light which permeates the glass, and instantly converting it into heat.

479. The two cylinders are connected by small pieces of thermometer-tubes, which keep them steady, with their faces parallel to each other, but turned in opposite directions, and all serve

to make the insulation as complete as possible The chambers are then connected by a small bent tube in the form of the letter U, having small bulbs near its upper extremities, and containing a little sulphuric acid, tinged with carmine. The instrument is supported upon a pedestal, having a vertical opening through the stem, to allow the glass tube to pass along it, and thus secure it from accidents.

480. A small scale divided into any number of equal parts is attached to each branch of the tube. In plate VII. fig. 1, ABCD and EFGH are the cylinders; A B, and FG, the plates of glass. CD, EFG, the ends shut up by the cylinder tin plates: the blackened paper is represented by the lines between A B, CD, and EH, FG. The other parts will be obvious from the mere inspection of the figure.

481. The accuracy of the instrument evidently depends upon the perfect equality of its two opposite ends. To ascertain if it be accurately constructed, place it between two steady flames, and move it nearer the one or the other, till the liquid in the tube remains stationary at the division of the scale at which it formerly stood. Turn it half round, without altering its distances from the flames, and, if the liquid remains stationary at the same division, the instrument is correct. To show the extreme delicacy of the instrument, place it opposite a single candle, and it will be sensibly affected at the distance of ten, twenty, or thirty feet, provided it be of sufficient diameter, whilst it will not be sensibly acted upon at the same distance by a mass of heated iron affording twenty times the quantity of heat. In order to cut off effectually the influence of mere radiant heat, I sometimes use screens composed of two plates of glass, placed parallel to each other, with a quantity of water interposed.

482. Place the instrument between any number of steady lights whose intensities are known: as, for example, between four wax candles opposite one end, and one candle opposite the other, and move the photometer till the fluid remains stationary at the division where it formerly stood, and it will be found that the distances are directly as the square roots of the number of candles; or, in other words, that the intensities of the lights will be inversely as the squares of the distances. If gas lights be employed having burners capable of consuming known quantities of gas in equal times, and the photometer be placed between them, so that the effect upon the air in each chamber shall be the same, it will be found that the quantities of gas consumed by each will be exactly proportional to the squares of the distances of their respective flames from the end of the photometer

483. This instrument seems well adapted for determining the relative quantities of light given out by the combustion of coal and oil-gas. Place the instrument as before between the two burners, and ascertain the relative intensities of the two lights, by squaring their distances from the adjacent ends of the instrument; ascertain the quantities of gas consumed by each of the burners in the same time; multiply these quantities by the squares of the respective distances, and

the product will be the relative quantities of light afforded by the gases. Let d be the distance of the coal-gas light, and d' that of the oil-gas light; and let q be the quantity of coal-gas consumed in a given time, and q' the quantity of oil-gas consumed in the same time, then the intensity of the coal-gas will be to that of the oil-gas q d2 to q1 d12.

484. To find the ratio between the quantities of light given out by the sun and that afforded by a common candle, place one end of the instrument opposite the sun, and bring the candle opposite the other end, till the fluid in the stem remains stationary at the original division; and the light given out by the candle, will evidently be to that given out by the sun, as the square of a few inches to the square of the number of inches contained in 95,000,000 miles, provided none of the sun's light had been absorbed in its passage through the atmosphere. The delicacy of the instrument is such that, if it be placed opposite the sun without a counteracting force, the light absorbed from the body will be so great, as to cause the liquid to move through a tube twenty or thirty feet long. By covering one end of the instrument and directing the other to various quarters of the sky, we can ascertain the relation between the quantities of light reflected from the atmosphere and clouds floating in those regions.

485. Though this instrument has some resemblance to professor Leslie's photometer, yet it is founded on principles essentially different: the one depending on the difference of the temperatures of the two bulbs, whilst the perfection of the other results from the equality of the temperature of the air contained in both chambers. The one has a scale a few inches long attached to one end of the vent tube, whilst the scale of the other is the distance between the two antagonist flames. The delicacy of the one is, from its very nature, combined within very narrow limits, whilst that of the other may be increased at pleasure.

486. Having thus furnished a brief outline of the most valuable new instruments, we cannot better conclude our general view of this important science than by furnishing our readers with a few experiments illustrative of the subject.

487. The cistulu is a machine or apparatus whereby small bodies are represented extremely large, and near ones extremely wide, and diffused through a vast space, with other agreeable phenomena, by means of mirrors, disposed by the laws of catoptrics, in the concavity of a kind of chest.

488. Of these there are various kinds, accommodated to the various intentions of the artificer some multiply the objects; some deform; some magnify, &c. The structure of one or two of them will suffice to show how many more may

be made.

with plane mirrors: in the lateral planes make round holes, through which the eye may be directed within the cells De

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of the box. The holes are to be covered with plain glasses, ground within, but polished, to prevent the object, in the cells, from appearing too distinctly. In each cell are to be placed the different objects whose images are to be exhibited; then covering up the top of the box with a thin transparent membrane, or parchment, to admit the light, the machine is complete.

490. For, from the laws of reflection, it follows, that the images of objects, placed within the angles of mirrors, are multiplied, and appear some more remote than others; whence the objects in one cell will appear to take up more room than is contained in the whole box. By looking, therefore, through one hole only, the objects in one cell will be seen, but those multiplied and diffused through a space much larger than the whole box; thus every new hole will afford a new scene: according to the different angles the mirrors make with each other, the representations will be different: if they be at an angle greater than a right one, the images will be distorted.

491. The parchment that covers the machine, may be made pellucid, by washing it several times in a very clear ley, then in warm water, and bracing it tight, and exposing it to the air to dry. If it be desired to throw any color on the objects, it may be done by coloring the parchment. Zahnius recommends verdigris ground in vinegar for green; decoction of Brasil wood for red, &c.

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492. Let us now, however, point out how to make a catoptric cistula to represent the objects within F it prodigiously multiplied, and diffused through a vast space. To construct this amazing apparatus, take a chest, as in M the above engraving, but without dividing the inner cavity into any apartments,

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or cells; line the lateral planes CBHI, BHLA, AL MF, &c., with plane mirrors, and at the foramina, or apertures, pare off the tin and quicksilver, that the eye may see through: place any objects in the bottom MI, as a bird in a cage, &c.

493. Here the eye, looking through the aperture h, will see each object placed at bottom vastly multiplied, and the images removed at equal distances from one another. Hence, with a large multangular room, in a royal palace, lined with large mirrors, over which were plain pellucid glasses to admit the light, it is evident