entire surface; he further gives some new experiments on induction, with a view of proving that when one surface is opposed as it were to itself, as in the case of the interior surface of a sphere, the inductive susceptibility of one-half the surface is reduced to zero. The phenomena of what the author calls, after Cavendish, electrical charge, he refers to some peculiar arrangement or disposition of the electrical particles on the surfaces of the several conductors, by which they exhibit a greater or less degree of excitement, as observable by the electrometer. The remaining portion of this paper is devoted to the laws and phenomena of electrical attractive force. The attractive force of a given surface under a given charge does not depend on the quantity of electricity, but on the number of attracting points called into operation by what is usually considered as the attracted body. Two circular discs of very light wood, of 5 inches diameter, being carefully prepared in a lathe, were divided into six concentric rings, including a central plate of about an inch in diameter. The attractive force on each pair of rings was determined by means of the electrical balance and carefully noted; the force was as the several opposed areas; and when the series was combined into one plate, the force was the sum of the forces of the respective rings; when the attractive forces of circular plates equal in area to the several rings respectively were examined, the force was the same as that exhibited by the two rings whose area was the same; hence it is inferred that whether the charge operates from the circumference or near the centre, the attractive force is the same. Two rings combined exhibit forces equal to the sum of the forces taken separately; and when the force is examined between the plates or the several rings and a plane circular area of large and continuous surface, the forces are no greater than that between two plates or rings of equal area. When the distances between the attracting surfaces or the quantities of the electrical accumulation varied, then the force was as the square of the accumulation directly, and as the square of the distances inversely. The author extends these experiments to spheres of different diameters. He had shown in a former paper, that, taking the attractive force to be as the areas directly, and as the squares of the A distances inversely, according to the expression Fx two points might be determined within the hemispheres in which all the force may be conceived to be collected, and to be the same as if proceeding from every point of the hemisphere. If Z=the distance of either point, qq' taken within the hemisphere, r-radius, and a distance between the near or what may be termed the touching points of the (2ar+a2)* -a ; and if A=the surface, we spheres, then we have Z= have Foc A 2 (a+2) When both hemispheres are equal, and distance =a variable, then we have also Fo 1 a(a+2r)* The author in a for mer paper had applied these formula to the limited induction of a sphere of an inch radius; he now extends the inquiry to spheres varying from an inch to 5 inches or more in diameter, and finds the results conformable to the formula. He gives a table containing the results of a series of experiments with four spheres whose areas regularly increased, and the radii of which were from 1 to 2 inches in diameter. These were examined by the electrical balance. They were first placed with the points qq, or centres of force as calculated for each at a constant distance of 11 of an inch, in which case the weights requisite to balance the force with a given number of measures of electricity were as the opposed areas, thus confirming the preceding results deduced with plane surfaces; when the distances were varied, the force varied as the squares of the distances between the centres of force, or according to the formula Foc 1 'a(a+2r)' With the view of further verifying these results, a set of plane circular plates in pairs, each pair equal in area to the areas of the respective hemispheres of the spheres, were submitted to experiment at the same constant distance 11, so as to cut the points ql, or centres of force of the spheres; the attractive forces were found precisely the same as that of the opposed spheres to which the particular plates belonged. The author has examined at various times and with very rigid attention, the several conditions under which electrical attractive force conforms to the law of force as deduced by Cavendish and Coulomb, and other eminent philosophers, and he finds this law true only for charged and neutral conductors of large inductive capacity; if either of the attracting surfaces have a narrow or limited susceptibility of inductive charge, then this law of force no longer obtains. If, for example, the attracting plates be taken as mere planes of small thickness, and even although they be charged with opposite electricity, still in changing the distances between them we do not obtain at all distances a law of force in the inverse duplicate ratio of the distance such as we have found to obtain in other circumstances. The force will be commonly in an inverse simple ratio of the distance. If the neutral or suspended plane be taken very thin and insulated, then little or no attractive force is observable under any circumstances. If we continue to increase its thickness, then, as the author has shown in former papers*, attractive force begins to display itself, and will approach a law of change in the inverse duplicate ratio of the distance as we extend its dimensions. When we give it unlimited electrical extension by placing it in communication with the ground, then the force is as the square of the distance inversely; but it is not always so, until we effect this extension perfectly. When all these sources of disturbance are duly considered, it will not be difficult to reconcile the many conflicting results arrived at by several eminent philosophers in past times, in their endeavours to investigate the law of electrical force, and explain how, without any defect in their experimental processes, such conflicting results might arise. Volta, for example, found electrical force to vary in a simple inverse ratio of the distance. M. Simon, of Berlin, an eminent philosopher, and eulogized by Gilbert as being "remarkable for his dexterity and careful manipulation" in this branch of physics, failed to verify Coulomb's result, although he employed a new and very delicate apparatus, by which the repulsive force between two spheres was very accurately and beautifully measured. In these experiments he found the force to vary as the distance inverselyt. * Phil. Trans. for 1834. † Poggend. Annal. for 1808, cap. 3, p. 277, and Ann. de Chim. vol. lxix. First Series. June 19, 1856. The LORD WROTTESLEY, President, in the Chair. The following gentlemen were admitted into the Society : John Carrick Moore, Esq. Edmund Potter, Esq. Henry Hyde Salter, M.D. The following gentlemen were recommended by the Council for election as Foreign Members : Wilhelm Karl Haidinger. Antonio Secchi. The following communications were read: I. "On Colour-Blindness." By WILLIAM POLE, Esq., F.R.A.S., The author's object in this paper is to state his own case of colourblindness, which he believes to be one of the most decided on record; to compare it with others of the same kind; and to show that the general phenomena attending this defect of vision are more uniform and consistent, as well as more easy of explanation, than is generally supposed. For general information on the subject, reference is made to a work lately published by Dr. Wilson of Edinburgh, entitled "Researches on Colour-Blindness," in which a great number of cases are fully described, and the optical and physiological theories of the defect carefully discussed. After stating reasons which justify a colour-blind person undertaking the investigation and description of his own case, the author gives a preliminary statement of his views in regard to the general theory and nomenclature of colours, adopting the ordinary hypothesis that red, blue, and yellow are the three primaries; a theory which, though it has been lately called in question, receives, it is considered, new support from the phenomena of the defect of vision under consideration. Dr. Wilson describes colour-blindness as of three kinds, namely1. Inability to discern any colour except black and white. This is very rare. 2. Inability to discriminate between the nicer distinctions of colour. This is so common as to be apparently rather the rule than the exception. 3. The third variety is the only one here treated of. Its outward manifestation is the inability to distinguish between many of the colours most marked to normal eyes, and its most complete form is what is called dichromic vision; being total blindness to one of the three primary colours. The description of a case of colour-blindness may either be confined to a statement of what may be called the symptoms of the malady, i. e. the effects it produces on the individual's judgment of colours; or it may go further, and endeavour to describe the positive nature of the sensations experienced, the causes, so to speak, of the outward symptoms observed. The first is the plan usually adopted, but the author combines both in the account of his own case. As regards the outward symptoms, he finds them very similar to those of other cases; and for the purpose of showing this similarity, he collects in an appendix the principal features of nearly forty published cases, and points out that as a general rule he can corroborate the whole from his own impressions, the points where they appear to differ being very few and exceptional. An abstract is then given of the symptoms exhibited, as collected from these cases. They are as follows: Blue and yellow are perfectly distinguished, and are the only colours seen in the spectrum. Almost all colour-blind persons think they see red, but it is frequently confounded with green (the most common mistake), VOL. VIII. Q |