smallest visual angle is about half a minute of a degree, and at a medium not less than two minutes. To most eyes, the nearest distance for distinct vision is seven or eight inches. The external shape of the eye is found to affect the sight. The eyes of some persons are naturally too convex; some acquire too great a convexity of the eye by close reading, which alters the shape of the crystalline humour; the image then becomes formed too soon, from the rays meeting in a focus before they reach the retina, unless the object be brought near the eye, in which case the image will be cast farther back. Shortsighted persons, therefore, use concave glasses to view objects at a distance, which diverge or spread the rays, and render the vision distinct. The eyes of persons in the wane of life generally become less and less convex, the image will not then be formed soon enough on the retina; convex glasses will, therefore, be used, by which means the rays of light are converged, and the image is clearly delineated. According to Sir I. Newton, Light is not a body of a homogeneous nature, but consists of rays of different kinds or colours,* each of which in passing from one medium to another differs in refrangibility. A ray of light is found to contain seven colours, red, orange, yellow, green, blue, indigo, and violet: these are called primary colours, but these seven are reduceable to three, red, blue, and yellow. If a hole be made in a shutter in a darkened room, and a ray of light be made to pass through a prism held obliquely and received on a screen at a proper distance, the ray will be found to be divided into the seven primary colours just named, and if the image of the rays (called a spectrum) be divided into 360 equal parts, the red will be found to occupy 45 of these parts, the orange 27, the yellow 48, the green 60, the blue 60, the indigo 40, and the violet 80. The rainbow which is formed by the refrac Colour is considered a property inherent in light, by which it excites different vibrations in the optic nerve, which being conveyed to the brain affect the mind with different sensations. According to Sir R. Phillips different colours are different perceptible effects on the optic nerves by the different forces or action of the rays, the red being the most forcible, and the violet the least. tion and reflection of the rays of the sun in drops of falling rain most beautifully exhibits the natural separation of the colours of Light. Of the seven primary colours the red is the least refrangible, and the violet the most refrangible. The strength of the red rays is visible on a hazy morning; the other rays being unable to penetrate the mist, the sun appears of a deep crimson colour. When all the rays are equally refrangible, light is said to be homogeneous, and when some rays are more refrangible than others, it is said to be heterogeneous. The colours of homogeneous light are not altered by refraction, and any thing viewed in homogeneous light will appear of the colour of the rays which fall upon it. Red lead or yellow ochre, for instance, in a homogeneous green light will appear green, and so would any other substance. The colours of bodies are said to arise from their disposition to reflect one sort of rays and to absorb the others; thus a red substance absorbs all the rays but the red which it reflects. The whiteness of bodies arises from their reflecting all the rays of light promiscuously, and the blackness of bodies from their absorbing all the rays of light thrown on them. That white is a compound of all the primary colours is prettily shown by painting on a circular board the seven colours in the proportion in which they are on the spectrum, and by whirling the board round with great velocity, the whole will be so blended together as to appear of a white colour which will be more or less perfect as the colours are more or less perfectly laid on. COLD. Effects of Cold-How produced artificially-Fatal results from exposure to extreme Cold, &c. COLD is that sensation which accompanies a transition of the fine vessels of the human body from an expanded to a contracted state. As heat is said to be caused by a particular motion of the particles of a hot body, so cold is its opposite, or the absence of such motion. Although contraction is the general result of cold, yet some bodies, under peculiar circumstances, expand as they cool: thus iron expands by heat, yet when melted it is found to expand in cooling. Water expands as it is heated and contracts as it cools, yet just before it begins to freeze it gradually expands again. The expansive force of water is most astonishing; it is capable of rending rocks or of bursting asunder very thick shells of metal. It has been found by experiment that the expansive power of a spherule of water of one inch in diameter in freezing, is capable of overcoming a resistance of 27,000 lbs. Cold may be produced artificially although it cannot be made to increase itself as heat will do. The greatest degree of cold that has been produced artificially has been eighty degrees below zero, or 0 on Fahrenheit's thermometer. Even snow and common salt mixed will sink a thermometer from the freezing point to zero, and if a little water be poured on a table in a warm room near a fire, and the vessel containing the mixture be put on it, it will become in a very little time completely frozen to the table. The effects of extreme cold are very surprising: rivers and lakes become frozen several feet deep: metallic substances blister the skin like red-hot iron: the air, when drawn in by breathing, hurts the lungs and excites a cough : even the effects of fire, in a great measure, seem to cease; and it has been observed, that though metals are kept for a considerable time before a strong fire, they will freeze water when thrown upon them. When the French mathematicians wintered at Tornea, in Lapland, the external air, when suddenly admitted into their rooms, converted the moisture of the internal air, into flakes of snow. The same circumstance occurred to our hardy seamen under Captain Parry when wintering in the frozen ocean. Extreme cold often proves fatal in those countries where the winters are very severe; thus 7000 Swedes perished at once, in the year 1719, in attempting to pass the Dofrafield mountains to attack Drontheim. But it is not necessary that the cold, in order to destroy human life, should be so very intense; it is only requisite to be a little below thirty-two degrees of Fahrenheit or the freezing point, accompanied with snow or hail from which shelter cannot be obtained. The snow which falls upon the clothes or the uncovered parts of the body melts, and by a continual evaporation carries off the animal heat to such a degree, that a sufficient quantity is not left for the support of life. In these cases the person first feels himself extremely chilled and uneasy; he becomes listless, unwilling to walk, or to use exercise to keep himself warm; and at last feels drowsy, sits down to refresh himself with sleep, but awakes no more. A striking illustration of the effect of cold is related by Captain Cook in an occurrence during a botanical excursion of Sir Joseph Banks and Dr. Solander among the hills of Terra del Fuego. The party, consisting of eleven persons, were overtaken by darkness during extreme cold; and Dr. Solander, who had more than once crossed the Dofrafield mountains, well knew that extreme cold, especially when united with fatigue, produces a torpor and sleepiness that are almost irresistible; he therefore conjured the company to keep in motion, at whatever pains it might cost them: "Whoever sits down," said he, "will sleep; and whoever sleeps will wake no more." Being thus admonished they set forward; but while they were still upon the naked rock, and before they had got among the bushes, the cold became suddenly so intense as to produce the effect they had so much dreaded. Dr. Solander was himself the first who found the inclination, against which he had warned the others, irresistible, and insisted upon being suffered to lie down. Mr. Banks entreated and remonstrated in vain; he lay down upon the ground, although it was covered with snow, and it was with great difficulty that his friend kept him from sleeping. Richmond also, one of the black servants, began to linger, having suffered from the cold in the same manner as the doctor. Mr. Banks, therefore, sent five of the company forward to get a fire ready at the first convenient place they could find, and himself with three others remained with the Doctor and Richmond, whom partly by persuasion and partly by force they made to proceed; but when they had gone through the greater part of the birch and swamp they both declared they could go no farther. When Richmond was told that if he did not go on he would in a short time be frozen to death, he answered that it was his desire to lie down and die. The doctor expressed his willingness to go on, but was desirous of taking some sleep first, although he had before told the company that to sleep was to perish. Mr. Banks and the rest found it impossible to carry them, and there being no remedy they were both suffered to sit down, and they almost instantly fell into a profound sleep. Soon after, some of the people who had been sent forward returned with the welcome news that a fire was kindled about a quarter of a mile further on the way. Mr. Banks then endeavoured to wake Dr. Solander, and happily succeeded; but though he had not slept five minutes he had almost lost the use of his limbs, and the muscles were so shrunk that his shoes fell from his feet; he consented to go forwards with such assistance as could be given him, but no attempts to relieve poor Richmond were successful. He, together with another black left with him, died. Several others began to lose their sensibility, having been exposed to the cold and snow for nearly an hour and a half, but the fire recovered them. PNEUMATICS. Air a compound body-Figure, Height, Weight, Pressure, &c. of the Atmosphere-Air the supporter of animal and vegetable life-Expansion and condensation of Air-Refractive and reflective power-The Air-PumpAir-Gun-Condensing Syringe-Barometer, &c. explained. * THIS science treats on the weight, pressure, elasticity, and other phenomena of that invisible fluid that surrounds our globe. It used to be supposed that the atmosphere consisted only of two distinct substances; air, and water in a state of vapour. But it is now known that atmospheric air is a compound of two distinct gases, oxygen, and nitrogen or azote, in proportion of about twenty-two parts of the former, to seventy-eight parts of the latter; it also contains about one part in one thousand of carbonic acid gas, and one part in seventy of aqueous vapour. There is also in the atmosphere hydrogen formed from the decomposition of water, and other gases, together * See the article Gaseous Bodies. |
