LIGHT AND HEAT. Photometric Measurements of the Magneto-electric Light. Determination of the Conductivity of Heat by Water. By J. T. BOTTOMLEY. On the Testings of Large Objectives. By HOWARD GRUBB, Master of Engineering, Trinity College, Dublin. In the testing of large objectives, when the corrections have been made to be very nearly perfect, a difficulty is sometimes felt in determining what, if any, corrections remain desirable, and also of determining in a simple way the amount of the desired alteration. For the chromatic aberration one plan often pursued by the optician is to slightly overcorrect the objective in the first instance, and then to separate the crown and flint (thus reducing the correction) until the best result is attained, when the amount of separation required becomes by a simple calculation a measure of the necessary alteration in the curves. This is an extremely useful practical arrangement; but unfortunately it is applicable to only one of the four possible errors, besides being troublesome and somewhat dangerous in the case of large objectives. The desirability of some simple plan of introducing, pro tempore, a small + or - effect of chromatic or spherical aberration, and of being able to accurately estimate the quantity of such, has been very apparent to me on several occasions; for I have frequently found the best judges of such matters differ in their estimate of final correction, and unable to agree thereon; and I have also often found a difficulty in satisfying myself that the best balance of corrections had been attained; whereas if it were possible to introduce a small amount, pro tempore, of + or correction, I could at once have perceived when I had overshot the mark. In fact the perfection of any correction in an objective means the best balance between two opposing aberrations, and (just as in all cases of ascertaining balances) it is difficult to determine the neutral point unless there be the power of trying on both sides. To effect this, in preparing for the trial of the great objective for the new Observatory at Vienna (of 27 inches aperture), I anı constructing four lenses or combination of lenses capable of being mounted between the objective and ocular, and with a considerable range of motion in the axis of the telescope : A. While it effects no other correction introduces a small amount of + chromatic aberration. B. Similarly introduces a small amount of chromatic aberration. + spherical aberration. The amount of any aberration introduced can be regulated by the position of the correcting lens in the pencil of rays. Now, knowing the construction of these combinations and their position in the pencil of rays from objective to ocular, the corresponding correction in the objective is an easily calculable quantity. Quite apart from the use to the optician I believe the comfort of these appliances will be much appreciated by those appointed as judges, particularly where, as in the case of the Vienna telescope, the testing of the objective forms part of the work of a Committee composed of a considerable number of Members. I have already experimented in this direction sufficiently to convince myself of the great value of this system so far as the correction for chromatic aberration is concerned; but I have not as yet experimented on the spherical aberration, nor am I so sanguine of its success. There seems another direction in which a possible advantage might be gained by use of these correcting combinations, viz. in the case of minute stars whose light is made up for a great part of rays from either end of the spectrum, more particularly the blue end. It seems highly probable that better definition of these stars could be obtained if a slight temporary adjustment could be made in the chromatic correction suitable for that particular part of the spectrum from which the predominant light of the stars proceeds. Of course it is to be understood that the corrections here spoken of and proposed to be dealt with by their correcting lenses are only the very final ones-in fact, when the objective arrives at that degree of perfection in which it is almost impossible to say whether any improvement can be effected or not. On Recent Improvements in Equatorial Telescopes. By HOWARD GRUBB, Master of Engineering, Trinity College, Dublin. The author referred to former papers read by him at the Brighton and Belfast Meetings of the Association on the same subject, and proceeded to describe1st. A method of conveniently reading the R.A. circle from the eye-end of the telescope. 2nd. A new simple but effective arrangement for slow motion in R.A. 3rd. A new and very much improved form of clamping arrangement for both polar and declination axes. 4th. And a new method of controlling the uniform motion driving-clock of the telescopes from an ordinary sidereal clock by an electric current transmitted once a second from the sidereal clock; by which arrangement the driving-clock can be kept going continuously without the possibility of accumulation of errors beyond a small fraction of a second. On a Method of Photographing the Defects in Optical Glass arising from want of Homogeneity. By HOWARD GRUBB, Master of Engineering, Trinity College, Dublin. The best practical method used for detecting in disks of optical glass defects arising from want of homogeneity is probably well known to many amateurs as well as to professional opticians. The disk of glass to be examined should be either itself polished to a convex form, or, if that be not convenient, it should be placed in juxtaposition with a piece of glass which is known to be perfect and of such form as will render the combination of the two of convex power. A small light (say gas- or candle-flame, or any sufficiently brilliant light with a small diaphragm in front, see fig. 1) is placed at some Fig. 1. little distance, and the eye is placed in the conjugate focus formed by the lens of this light. The disk of glass should then appear brilliantly illuminated; but if the pupil of the eye is drawn slightly to one side, so that the pencil of light falls upon only one half of the pupil, immediately and most distinctly almost any want of homogeneity is easily seen. 1 say "almost any want of homogeneity," because, with one exception, I believe any kind can be detected; but I have met, very rarely, instances of one peculiar class of this defect which it is not possible to detect till the disk is actually worked into an objective: this happens when a slight gradual change of density occurs between two portions of the disk with no abrupt line of separation between. Now this process, though a very simple one to a practised eye, is by no means so to an uneducated one; and I have often desired a method by which I could graphically represent those faults so that I might be able to communicate to others my ideas as to their exact forms and appearance, position in the disk, and so forth, and also to form a record of them. This, by a very simple contrivance, I have succeeded in doing, and I am now able to photograph these defects in optical glass with perfect certainty. A glance at the diagram will suffice to show the principle by which this is effected. Fig. 2. The eye in the first instance (that of eye observation, fig. 1) is replaced in the second case (fig. 2) by a photo-camera; and, with a little care in adjusting the image of diaphragm illuminated by a lamp on the diaphragm of photo-lens, very excellent photographs can be obtained. In fact the stop of the lens replaces the pupil of the eye, the photo-lens the crystalline lens, and the sensitized plate the retina. The defects arising from want of homogeneity in optical glass may be divided into three classes: 1. Threads, or fine seams of some different quality of glass passing through the otherwise homogeneous disk, sometimes insignificant, sometimes long, but very rarely of any width. These are of but little importance. 2. Veins, or syrupy bands. These are portions of glass of differing and various densities not properly amalgamated together. Their appearance is that produced by adding a strong syrup solution to water. The forms of these veins are sometimes very fantastic. This form of defect is very detrimental to the proper performances of the glass. 3rd. Sometimes, but very rarely (only four times in my experience), have I met with disks of glass having a density slightly different in different parts, without any well-defined line of demarcation between the different parts. This is most destructive to its performance as an objective, and a most dangerous fault; for whereas in the two former cases the defects can be easily detected and even photographed, this third defect defies detection until the disks be formed into an objective. It is fortunate for opticians that this last defect is of such rare occurrence. The extreme usefulness of this simple device for photographing the defects in optical glass is self-evident. In the first place, faults can be detected by those whose eyes have not been sufficiently educated to perceive them by the old method; a record can be made of any remarkable defects; their appearance and form can be graphically represented and described; and, lastly, it can be ascertained by this process whether the veins are closer to one or other surfaces and are capable of being removed by grinding, a point which is very difficult indeed to ascertain otherwise. This last information is obtained by photographing the faults and then grinding off a small quantity, and rephotographing and comparing the photographs to see if any parts have disappeared. Many other useful purposes seem to be too self-evident to require mentioning. On the Decrease of Temperature with Height on the Earth's Surface. If the air were perfectly still, the temperature at any point in the atmosphere would depend on its density, the heat absorbed from the solar rays, the heat obtained by convection from the earth, and the losses of heat by radiation. Of these the first has been almost exclusively considered. This is especially so in all investigations for the ascertainment of heights by the barometer. The exclusion of the other causes of variation of temperature with height may be admissible in considering the condition of a vertical column of air resting on a horizontal plane; but the problem assumes a very different character when the decrease of temperature with height along a very gradually sloping surface is considered. Such a surface is constantly communicating its temperature by convection currents to the overlying air, and the temperature of this air will depend on the extent, form, and physical properties of the underlying surface. If we suppose a flat plain on the level of the sea, an observer in a balloon at a height of 1000 feet would find the temperature almost unaffected by convection and dependent upon density. If, now, a steep mountain is superimposed on the plain and reaching to the observer, the conditions become altered. If a mountain of a gradual slope be superimposed, the alteration will be still greater; and if the entire plain were elevated up to 1000 feet so as to form an extensive tableland, the change of conditions would be very remarkable. It follows that the law of variation of temperature with height above the level of the sea cannot be considered as uniform. The decrease is most rapid in going up through a vertical column of air, as in balloon ascents. It is slower along mountain sides, and slowest along gradually sloping plains or tablelands. From an examination of the records of many observations, it appears that the decrease of temperature in balloon ascents is nearly one degree Fahrenheit for 300 feet, while for tablelands it is so slow as from 500 to 800 feet for one degree. The author referred to a number of observations made in different countries confirming the general conclusions to which he has been led. On the Distribution of Temperature over the British Islands. The author referred to his former researches on the distribution of temperature over islands surrounded by heat-bearing currents and his demonstration that many of the isothermal lines in such islands must necessarily be closed curves. He had originally illustrated his conclusions by the results of observations taken in the British Islands, and the isothermal lines laid down from such observations were found to be in perfect harmony with the law he had proved. In order to render this manifest he tabulated together the temperature of each, stating its latitude, longitude, height above the sea, and horizontal distance from the nearest sea coast. The actual temperature of any place is affected by all of these elements. In laying down the isothermal lines the actual temperatures unaltered by any so-called correction for height were always employed. The stations were arranged according to temperature, and thus isothermal groups were immediately discovered. If the more recent collection of temperature results for the British Isles compiled by Mr. Buchan in the Journal of the Scottish Meteorological Society be treated in this way and the arbitrary and erroneous addition of 1° per every 300 feet in height be omitted, his results will conform to the law enunciated by the author. *See Brit. Assoc. Rep. 1857, pt. 2, p. 30; Atlantis, i. 1858, pp. 396-413; Phil. Mag. xvi. 1858, p. 241; Royal Society Proc. x. p. 324; "On the Laws which regulate the Distribution of Isothermal Lines," Atlantis, ií. p. 201; American Journal of Science, xxvii. p. 328. Copies of the temperature-maps are also partly reproduced in Report of Horticultural Congress at London in 1866; Journal of the R. Dublin Society, vol. for 1870-71 Report of the Commission on Oyster Fisheries, 1871. Sur les Usages du Revolver Photographique en Astronomie et en Biologie. By Dr. J. JANSSEN. Photographies du Passage de Vénus à Kobé. By Dr. J. JANSSEN. Sur le Mirage en Mer. By Dr. J. JANSSEN. On Solar Photography, with reference to the History of the Solar Surface. By Dr. J. JANSSEN. On the Eclipse of the Sun observed at Siam in April 1875. On Rotation of the Plane of Polarization by Reflection from a Magnetic Pole. By JOHN KERR, LL.D., Mathematical Lecturer of the Free Church Training College, Glasgow*. In these experiments a beam of light is polarized by a first Nicol, reflected regu larly from the end of an electromagnetic core of soft iron, and analyzed by a second Nicol. The magnetic force is concentrated intensely upon the mirror by means of a massive wedge of soft iron, which is separated from it by a narrow chink. The light is incident upon the polar mirror at an angle of 60° to 80°; the plane of polarization coincides with the plane of incidence; and the two Nicols are exactly crossed, so that the reflected light is extinguished by the second Nicol. First Experiment.-When the iron mirror is intensely magnetized as N. pole or S. pole, the light is distinctly restored from pure extinction, to disappear at once when the circuit of the magnetizing current is broken. Second Experiment.-The first Nicol is turned from its initial position through an extremely small angle-(1) to the right, (2) to the left (from the point of incidence on the iron mirror as point of view), so that the reflected light is restored very faintly through the second Nicol. When the mirror becomes an intense S. pole, the effects of rotations (1) and (2) are strengthened and weakened respectively; on the contrary, when the mirror becomes an intense N. pole, the effects of rotations (1) and (2) are weakened and strengthened respectively. In the two remaining experiments the optical effects of the preceding rotations (1) and (2) and of magnetizations S. and N. of the mirror are compensated separately. The compensator is a slip of plate-glass, held in a standard position between the mirror and the second Nicol and strained by the hands. The angle of incidence is about 75°. Third Experiment.-The first Nicol is turned from its initial position through an extremely small angle-(1) to the right, (2) to the left, so that the light is faintly restored from extinction by the second Nicol. The effects of displacement (1) and (2) are compensated, down to pure extinction, by tension and compression respectively. Fourth Experiment.-A repetition of the first, with addition of the compensator. The effects of magnetizations S. and N. of the mirror are compensated, down to pure extinction, by tension and compression respectively. The case of perpendicular incidence was tried carefully, but gave no good effect, the arrangements being comparatively imperfect. From the facts observed, it follows evidently that when a beam of plane polarized light is reflected from a magnetic pole, the plane of polarization is turned in the process of reflexion-to the left by a south-seeking pole, to the right by a north-seeking pole; so that in this * A full account of this investigation is given in the Phil. Mag., May 1877. |