the arm N'. The latter adjustment is rendered permanent by tightening the screws S. If the apparatus be in good adjustment, the reflected image of the luminous object should remain on the horizontal wire, at whatever angle the telescope is inclined to the object; and the process of adjustment ought to be repeated until this is the case. It will afterwards be generally sufficient to adjust the instrument by means of the collar R. The apparatus being adjusted, the principal section of the prism, and consequently the plane of reflexion between the glasses, is brought into coincidence with the plane of the divided limb by means of the ring E F. The refracted image will now be upon the horizontal wire of the telescope; and by moving the ring G H, and following the motion of the refracted image, a point will be found where it stops and begins to move in the opposite direction. By means of the tangent screw, the cross-wires are made to intersect the image at this point, at which the deviation is a minimum, and the angle is read off. The telescope is then turned round, until the reflected image intersects the cross-wires, and the angle is read off. The direct bearing of the object being then taken, the differences between it and the other observed angles will give the deviations of the reflected and refracted rays, which must be corrected for the parallax due to the distance of the prism from the centre of the instrument.* From these angles the index of refraction is easily calculated. Let = the observed deviation of the refracted rays; the angle of the prism; and g=the observed deviation of the reflected rays. Then, since the prism is in its position of minimum deviasin(+); which expressed in terms of the observed tion, μ = sin *If the distance of the observed object be great, the correction, in seconds, to be added to an observed angle & will be nearly r sin a ; where d is the distance of the observed object, and r the distance of the prism from the centre of the theo doli e. It is obvious, that this method of determining refractive powers is particularly well adapted for fluid substances. The fluid may either be contained in a hollow prism made of parallel plate glass, or a few drops may be retained by capillary attraction between the glasses of the instrument; in either case, the angle of the fluid prism will be ascertained in the manner already shown. From a considerable number of observations, I have found that the method which has now been described, affords considerable facilities in determining refractive powers. The construction of the apparatus makes the adjustment of the surfaces of the prism quite independent of each other. For the rings E F, G H (fig. 6), and the face of the prism in contact with the glass A B, remain constantly perpendicular to the divided limb; and the face in contact with the glass C D, is directly adjusted by the motion of the ring EF. The application of glass-plates to the surfaces of the prism also greatly facilitates the process, inasmuch as it renders it unnecessary to have the faces of the prism so highly polished as would otherwise be indispensably requisite. In fact, it is generally sufficient to grind the surfaces smooth with fine emery, when the fluid by which the prism is retained between the glasses will render it perfectly diaphanous. Sir David Brewster, who seems to have been the first to avail himself of this method of making prisms, observes, that, in measuring the refractive and dispersive powers of bodies that are incapable of receiving a good polish, " by cementing upon the two refracting planes pieces of parallel glass with a fluid of nearly the same refractive density, substances like horn, rock salt, and several of the gums, may be rendered perfectly transparent."* In alluding to the circumstance, that the prism is included between two plates of glass, by whose inclination its refracting angle is ascertained, I am led to anticipate some objections to the accuracy of the process that may suggest themselves to the reader. It may be thought, that not only will the refractive power of the glass-plates vitiate the result, but also that a source of *Treatise on New Philosophical Instruments, p. 279. error will arise from the glasses not applying with sufficient accuracy to the faces of prism. No appreciable error need be apprehended from the refractive power of the glasses, provided they be of the kind used for making the mirrors of sextants; and should any notable imperfection exist, it will manifest itself by the separation of the images reflected from the two surfaces of the defective glass. With reference to the other source of error, I may refer to the results of actual observation, to shew within what narrow limits it may be confined; and in doing so, I have to acknowledge the kindness of Mr William Nicol, Mr John Adie, and Mr Alexander Bryson, to whom I am indebted for the use of most of the prisms whose angles have been determined by this method. The angle of a flint-glass prism, belonging to Mr Adie, was taken five times, and the greatest and least result differed by only 50′′, the object observed being a street lamp nearly 400 feet distant. The angle of the same prism was then determined by a double observation on the turrets of Trinity Chapel, Deanbridge, at a distance of about of a mile, and the result differed from the mean of several accurate experiments made some years ago by Mr Adie, by 30". A plate-glass prism, examined in the same way, gave a result differing from Mr Adie's by 50", and the refractive indices of these prisms differed from Mr Adie's results only in the fourth place of decimals. A similar agreement exists between many other observations I have made on different substances and the results obtained by former observers, so that I feel quite satisfied with the practical efficiency of the process. EDINBURGH, 4 DUKE STREET, 10th June 1843. On Solar Radiation. The experiments mentioned in the text (page 215), referred to a curious inquiry which has occupied my attention for some years, namely, the loss of force which the sun's rays experience in passing through the earth's atmosphere. It might VOL. XXXVI. NO. LXXI.- -JAN. 1844 H seem, at first sight, an impossible task to determine the comparative measure of the sun's heat, in the state in which it arrives at the earth's surface, and that which it would have attained were the atmosphere wholly removed. Some approximation to such a result has, however, been obtained by a very simple though indirect method. The thickness of air traversed by a sunbeam is, of course, least when the sun is vertical, and greatest when he is near the horizon; at intermediate elevations the heat is intermediate. Now, by comparing the thermometric effect of the sun's rays (which is the object of the actinometer), at several different thicknesses of atmosphere, the law of extinction is approximately found, and an inference is made as to what the intensity would be when the thickness of the atmosphere is nothing. This inference will be proportionally more accurate as the observations are pushed to a less thickness of interposed air; and I have shewn in the paper already referred to,* that the previous estimates had greatly underrated the intensity of the unimpaired solar-beam, and had also underrated the absorptive power of the atmosphere, owing to the observations on which they were founded having been made only when the sun-beam had already traversed a great thickness of air, when the law of absorption is very different from the law at small thicknesses. Now, to obtain observations of solar heat at small thicknesses, we must, in the first place, ascend in the atmosphere, and also use the sun's rays when his elevation is greatest, that is, near the solstice. I mounted the Cramont in hopes of prosecuting these experiments, when the sun had still 21° of northern declination, and after having left below me a thickness of 9000 feet of the densest part of the atmosphere. Unfortunately, as we have seen, these delicate experiments were prevented by indifferent weather. It will probably surprise many persons to be told, that even when the sun's rays shoot vertically through a pure atmosphere, as between the tropics, they lose in their passage (owing to the opacity of the air) very nearly half their Phiosophical Transactions for 1842, page 225, intensity. The intercepted heat goes, of course, to warm the air. The object of this note is, however, to record a different set of observations, performed with an instrument of inferior delicacy to the actinometer, but still capable of yielding very remarkable results,-I mean Leslie's photometer. Its principle may be briefly described as measuring the difference of the heat absorbed by a dark and clear thermometer-ball. It is well known that this instrument gives, on some occasions, results which appear highly paradoxical, but which, if consistent, require to be explained, and ought, therefore, to be distinctly established. My observations with it were directed to two points. 1. To ascertain the effect of the presence of a coating of snow on the ground, in magnifying the apparent Solar Radiation. To this effect has been ascribed the extraordinary force of the sun's rays observed in arctic climates, and also some singular variations from one season to another, supposed to depend on the presence of snow on the ground.† Now, the few experiments which I obtained before breaking my instrument last summer (1842), gave me the following most striking results : Surrounded by grass, and exposed to direct sunshine, the photometer indicated, Exposed upon snow instead of grass, it rose to 78° 121° The whiteness of the snow is all important in this respect; dirty snow produces comparatively little effect, and so does ice; thus, The photometer exposed on a dirty part of the Mer de Glace, stood This action fully explains the intensity of the effect of fresh snow upon the eyesight. I have myself found that exposure for several weeks to the moderate glare of sun-light reflected from a glacier surface, produced little effect upon me, whilst Phil. Trans. as above. ↑ Edin. New Phil. Journal 1841. A paper by Dr Richardson, with remarks. |