THE PRESENT SERIES of the POPULAR EDUCATOR is now completed. It was our intention to have closed it with a Supplement to Volume V., consisting of some 200 pages. But the difficulty of completing so many subjects in that limited space, together with urgent appeals from numerous Readers and Correspondents, induced us to postpone its termination till the completion of another Volume, uniform with the five preceding. This will explain the continuity of the paging from the commencement of the Fifth Volume to the close of the work. The appearance of uniformity will be greatly preserved by having the entire Series bound in three double volumes. In presenting these Six Volumes to the Public, we may confidently call them an EDUCATIONAL CYCLOPEDIA ; comprising a vast amount of solid and useful information in a popular form, and at a price unprecedented even in the present age of Cheap Literature. The highest encomiums have been bestowed upon our labours by a large portion of the Public Press, by learned Professors, by Teachers of Youth, and by a host of Students who have profited by our publication. Every post has brought us numerous expressions of gratitude for the seasonable and valuable aid we have rendered, and of deep regret that we have brought our labours to a close. We take our respectful leave of our friends, thanking them sincerely for the assurances they have given us that we have not laboured in vain. PAGE 434 XXXI. 453 XXXII. XXXIII. 513 XXVIII. La vierge aux ruines. Sections I., II., III., IV., 533 551 .... 593 ..... 599 615 632 XXXIV. Section IV., with exercises, etc. Le vaisseau en péril, with exercises, etc. ........................................ 647 527 LESSONS IN GEOLOGY. Classification of Rocks; Permian Formation 555 Lithological Character of the Coal Rocks 587 on the Coal Measures LXI. Old Red Sandstone, or "the Devonian Formation; Lithological Character of the Devonian; Organic Remains of the Devonian; Some Geological Phenomena of the Devonian Period LESSONS IN GREEK. XLVII. Formation of Words; Verbs; Compounds; Recapitulation... 617 651 716 421 XLVIII. Invariable Words 437 XLIX. L. 510 Syntax; Preliminary Explanations; Subject; ...... 450 ........ 465 XXXIII. Percentage; Commission; Brokerage; and Stocks LI. Voices of the Verb... ................ 479 544 LII. Tenses of the Verb..... 499 XXXIV. Interest; Compound Interest.. Moods XXXV. Discount; Bank Discount; Insurance; XXXVI. Life Insurance; Profit and Loss 606 640 XXXVII. Analysis LIV. Enlargement of Simple Sentences....... LV. Attributives; the Demonstrative Pronouns; the Article 546 576 LVI.. Attributive Words with Substantive Import; Enlargement of the Predicate.. LVII. ,, the Single Accusative LVIII. The Predicate with a Double Accusative...... LIX. Import and Use of the Dative ........... .... 603 641 674 701 736 XIX. .......... XX. Edward Gibbon ...................... XXI. Oliver Goldsmith.. ........................ XXII. William Cowper ........................ 764 .798 LX. LXI. LXII. Interrogative and Imperative Sentences ........ .............. 747 XXXVII. Irregular Verbs ending in "ere short; "Verbs ending in cere 448, 467 481 XXIX. Lydia Huntley Sigourney. ......... 811 XXXVII. XXXIX. XL. Verbs ending in dere 795 530 Verbs ending in urre, contracted from cere; Verbs ending in gere 561 Verbs ending in lore; Verbs ending in mere; Verbs ending in pere.. Vebs ending in ggere; Verbs ending in gliere; Verbs ending in guere.. 593 627 .... 432, 447 462 V. Compound Vowels Verbs ending in orre; Verbs ending in rere; Verbs ending in tere; Verbs ending in arre; Verbs ending in vere.... 659 497 VI. Diphthongs; Nasal Vowel Sounds; Nasal Diphthongal Sounds 524 VII. Liquids; General Rule for Pronouncing and Reading French XLVII. Impersonal Verbs; Participle; Adverb XLVIII. Preposition; Conjunction; Interjection 559 XLIX. Syntax: Of Articles XLIV. Table of Terminations of the Verbs in isco XLV., XLVI. 676 688, 703 .... 722 739 762 L. Of Nouns, Adjectives, Comparatives and Superlatives, Numerals.. 766 LI. LII. Of Personal, Possessive, Demonstrative, Relative, and Interrogative Pronouns.. 786 Of Indefinite Pronouns; of Verbs 801 LIII. Of Participles, Adverbs, and Conjunctions.... 813 LESSONS IN MORAL SCIENCE. I. Conscience, or the Moral Faculty; The Moral Faculty, Original and Universal; A Moral Faculty being supposed, whether its Dictates are Uniform? How far all Men are agreed in their Moral Judgments II. Whether Conscience is the same as the Understanding, or a Faculty different from and independent of it; Moral Sense compared with the Taste; Moral Obligation; Supremacy of Conscience III. Whether we always do right by obeying the Dictates of Conscience? Whether there is in the mind a Law or Rule, by which Man judges of the Morality of particular Actions? The Moral Feeling which accompanies every Moral Judgment; Belief in God, as connected with the Operation of Conscience. IV. Moral Agency, and what is necessary to it; Man a Moral Agent; Man not under a Fatal Necessity V. Man's Direction and Government of his Actions, and his consequent Responsibility; Objec tions to the Uniform Influence of Motives; Summary View of Liberty VI. The kind of Indifference which has been considered essential to Free Agency; Whether Men are Accountable for their Motives, or whether Desires and Affections which precede Volition have a Moral Character? The Division of Motives into Rational and Animal; Whether Morality belongs to Principles as well as Acts, or is confined to Acts alone?.. VII. Moral Habits; Nature of Virtue VIII. Different Hypotheses..... IX. Whether Virtue and Vice belong only to Actions? 661 X. Author of our Being, considered in Relation to Moral Science ON PHYSICS, OR NATURAL PHILOSOPHY. No. LIII. (Continued from page 396.) THE EYE CONSIDERED AS AN OPTICAL INSTRUMENT. Insensible part of the Retina.-The retina is not equally sensitive in every part, as is proved by the following experiment of Mariotte. Let two black spots be made on white paper, at a distance of from half an inch to an inch from each other. Then, when the paper is brought very near the eye, let the right eye be fixed upon the left spot without preventing it from seeing the other. If the paper be now slowly withdrawn, the right spot will disappear for a time, but reappear soon afterwards if the paper is still further removed. The same thing will happen if the right spot is looked at with the left eye. Mariotte has remarked that at the moment when the spot ceases to be visible, its image is projected upon the insertion of the optic nerve in the interior and lower part of the eye. This insensible part of the eye is called punctum cæcum, or the blind point. Continuance of the Impression on the Retina.-On whirling round a lighted coal with rapidity, we perceive a sort of band of continuous fire. Similarly, the rain which falls in drops, appears like liquid threads in the air. These appearances are owing to the fact that the impression produced by objects on the retina remains after the object is removed or replaced by another. The duration of this continuance varies according to the sensibility of the retina and the intensity of the light. M. Plateau of Brussels has discovered, by various methods, that it is on the average about half a second. The impressions of colours as well as forms remain after the removal of the objects that produce them, for if we divide a circle into sectors and paint them different colours, on turning it round, the colours mix and produce the sensation of the colour which would be formed by their mixture. Thus blue and green produce the sensation of green; yellow and red that of orange, blue and red that of violet; and the seven colours of the spectrum that of white, as is shown by Newton's disc. There are several curious apparatus, the effects of which are explained by the continuance of the impressions upon the retina. Such are the thaumatrope, the phenakisticope, the kaleidophone, and Farraday's wheel. Accidental Images. If a coloured object be placed upon a dark ground and looked at attentively for some time, the eye soon becomes wearied and the intensity of the colour grows feeble. On directing the eyes to a white piece of paper or on the ground, we perceive an image of the same form as the object, but of a complementary colour; that is to say, a colour which would form white if it were combined with that of the object. In the case of a green object, the image is red, and vice versa; if the object is yellow, the image is violet. These coloured appearances were remarked by Buffon, who gave them the name of accidental images or colours. Accidental colours continue for a length of time, proportioned to that during which the object was observed, and to the intensity of the light upon it. Generally speaking, they do not disappear gradually and without interruption, but present alternate disappearances and reappearances. It is well known also that if, after having looked attentively at a coloured object, we close the eyes rapidly, and as firmly as possible, so as to exclude the light, and even screen them from the light by means of a thick piece of cloth over them, the accidental images still appear. Various theories have been proposed to account for the phenomenon of accidental colours. That of Darwin is deserving of mention. He thinks that the part of the retina which is wearied by one colour, becomes insensible to the rays of that colour, and is only capable of impressions of the complementary colour; also, that this part of the retina spontaneously assumes an opposite mode of action, which produces the sensation of the complementary colour. The first part of this theory does not explain the appearance of accidental colours even in darkness, and the second part is merely a statement of the phenomenon of accidental images. YOL. V. Irradiation is a phenomenon in which white objects, or those of a bright colour, when seen on a dark ground, appear larger than they really are. The reverse takes place with a black object on a white ground. It is thought that irradiation arises from the circumstance that the impression on the retina extends more or less beyond the outline of the image. The effect of irradiation upon the apparent magnitude of the stars is very perceptible, and they may thus appear several times larger than they really are. According to the researches of M. Plateau, irradiation varies considerably in different persons, and even in the same person on different days. This philosopher has also shown that irradiation increases with the brightness of the object and the length of time it is observed. It is perceptible at all distances, is increased by divergent lenses and diminished by convergent ones. Accidental Halo. Contrast of Colours.-Colours which instead of following the impression of an object like accidental colours, appear round the object itself when attentively looked at for some time, are called accidental halos. The impression of the halo is the reverse of that of the object; that is to say, if the object is distinct, the halo is obscure, and vice versa. Contrast of colours is a reciprocal action which takes place between two colours nearly allied, and by virtue of which each of them assumes the complementary colour of the other. This contrast was observed by M. Chevreul, who profoundly investigated the subject, with a view to ascertain the laws of the phenomenon. It is attributable to the reciprocal action of the accidental halos above mentioned. M. Chevreul found that on red and orange being placed side by side, the red inclined to violet and the orange to yellow. If the experiment be made upon red and blue, the red inclines to yellow and the blue to green. With yellow and blue, the yellow passes into orange and the blue to indigo, and similarly with many other combinations. It is needless to remark how important is the bearing of this subject upon the manufacture of cloth, carpets and other coloured articles. Those who would wish to be successful in combining colours must understand the principles of the effect of contrast. The Eye not Achromatic. It was long the custom of philosophers to attribute to the human eye the property of perfect achromatism, but this notion cannot be admitted without qualification after the various experiments of Wollaston, Young, Fraunhofer and Muller. Fraunhofer observed that in a telescope with two glasses a very fine thread placed inside the instrument is distinctly seen through the eye-piece when the telescope is illuminated with red light only, but ceases to be visible, if, without altering the position of the eye-piece, the telescope is illuminated with light of a violet colour. To see the thread again, it is necessary to diminish the distance between the two glasses, much more than is required by the refrangibility of violet light. Hence it is evident that part of the effect is due to the aberration caused by the refrangibility of the eye. Muller found that, on looking with a single eye at a white disc on a black ground, the image is clear when the eye is adapted to the distance of the disc, that is to say, when the image is formed on the retina. But he observed, that if the eye is not adapted to this distance, that is to say, if the image is formed at a distance either in front or at the back of the retina, the disc appears to be surrounded with a very narrow blue band. He concluded from this and other experiments that the eye is achromatic as long as the image is received from the focal distance, or as long as the eye is adapted to the distance of the object. It is not yet known what is the precise cause of this apparent achromatism of the eye, but it is generally attributed to the delicacy of the pencils of light which pass through the aperture of the pupil, and to the fact that the rays being of various refrangibility, and meeting the media of the eye almost perpendicularly, are very little refracted, and hence the dispersion is not perceptible. As to spherical aberration, we have already seen how that is corrected by the iris, which is a real partition, arresting the marginal rays that have a tendency to go beyond the crystalline, and only suffering those to pass which are nearest the axis. Short Sight and Long Sight.-The usual cause of short-sightedness is a too great convexity of the cornea or crystalline. The eye being then too convergent, the focus instead of being 131 on the retina falls before it, so that the image is confused. This defect is remedied by means of divergent glasses, which, by removing the rays from their common axis, throw back the focus to the retina. The habit of frequently looking at small objects, and making microscopical observations, has a tendency to bring on short-sightedness. This defect is common among young people, but diminishes with advancing years. Long sight is the opposite of what we have just been considering. People having this affection see distant objects very well, but cannot clearly distinguish those that are near. The defect arises from the insufficient convergence of the eye, in consequence of which the image of objects close at hand is formed beyond the retina. But if the objects be removed to a distance, the image will approach the retina; and when they are at a suitable distance, the image is formed exactly on this membrane and the person sees clearly. Long sight is corrected by means of convergent glasses, which draw together the rays before they enter the eye, in consequence of which, if the convergence be suitably chosen, the image is brought exactly to the retina. Till within a few years it was customary to make use of none but bi-convex glasses for long-sighted people, and bi-concave ones for short-sighted people. Wollaston was the first to propose changing these glasses for concave-convex lenses c and F, fig.288, arranged in such a manner that their curvature corresponds to that of the eye. These glasses, enabling one to see objects which are far from the optical axis more clearly, are called periscopic (from TeρiorоTέw, I look round). Double Sight is an affection of the eye which causes it to see two objects instead of one. In general, the two images are almost entirely one over the other, and the one is much more apparent than the other. Double sight may result from an inequality in the size of two eyes, but it may also affect a single eye. This last affection is undoubtedly owing to some defect in the formation of the crystalline, or other parts of the eye, which causes the luminous pencil to become forked and form two images on the retina instead of one. A single eye may also be affected with triple sight, but in this case the third image is extremely feeble. Inability to discern Colours.-Some people are unable to distinguish colours, or at least some particular colours. Such persons can discern the outline and form of objects very well, and also the bright and dark portions, but nothing more. One who was thus affected had painted a landscape, in which the earth, the houses, the trees, and the figures were all blue. On being asked by a friend why he had not given each object its proper colour, he answered that he wished to make the colour of the picture accord with that of the furniture in his room; yet this was red. This defect is sometimes called Daltonism, because Dalton, who has carefully described it, was himself affected with it. SOURCES OF LIGHT, AND THE ACTION OF LIGHT ON PLANTS. Various Sources of Light.-The various sources of light are the sun, the stars, heat, chemical combinations, phosphorescence, electricity, and meteorological phenomena. The origin of the light emitted by the sun and stars is unknown. We only know that the inflamed substance with which the sun appears to be surrounded, is gaseous, because the light of that luminary, like that which is emitted by gaseous substances when inflamed, affords no trace of polarisation when viewed through instruments constructed for the purpose of detecting polarisation. As to light developed by heat, according to M. Pouillet, bodies begin to be luminous in darkness at a temperature of about 1350 or 1400 degrees Fahrenheit, and beyond that, the light they emit is brighter in proportion to the increase of heat. It is through the high temperatures accompanying many chemical combinations that these last occasion a disengagement of light. Such is the cause of the artificial light employed to represent lightning, for, as we have seen, flames are nothing more than gaseous substances heated to such a degree as to become luminous. As bodies become luminous at a high temperature, caloric seems to be then transformed to light, which would favour the idea, that these two agents ought to be referred to one single cause, especially if we bear in mind that the luminous rays are generally accompanied by calorific rays. But still the identity is not complete, for we know several substances which can be made to give out light in darkness without heat, or at least without any great degree of heat. Bodies of this sort are called phosphorescent, because this property is especially apparent in phosphorus. There are cases in which the phosphorescence is accompanied by a slow chemical action. This is true of phosphorus, certain vegetable or animal substances, as e. g. wood in a state of decomposition, and fish, especially herrings in a putrefied state. In other cases phosphorescence is developed under the influence of a high temperature. For instance, if we heat powdered fluor-spar to about 700 or 900 degrees Fahrenheit, it becomes suddenly luminous and gives forth a bright bluish light. Certain substances become luminous under the influence of solar light, It is thus that the diamond and several other minerals, having been exposed to the rays of the sun, for some time afterwards appear luminous when carried into a dark place. Fluor-spar, diamond, and white marble have the property of acquiring phosphorescence under the action of several successive discharges of a powerful electric battery. Lastly, phosphorescence is very intense in certain insects, especially the glowworm, which varies its light at will. In many countries, and especially tropical regions, the sea is often covered with a bright phosphorescent light, caused by zoophytes of extreme minuteness. These animalcules shed so subtle a luminous matter, that Messrs. Quoy and Gaimard, when on a voyage near the equator, having put two of them in a bottle of water, immediately saw the whole mass become luminous. Some philosophers, observing that in many cases the causes of phosphorescence are the same as those which develop electricity, and that feeble electric light strongly resembles that of phosphorescent bodies, infer that phosphorescence is owing to an electric cause. Action of Light on Vegetables.-Light exercises various kinds of action on vegetables. 1. It is under the influence of the solar rays that the green parts of plants acquire the property of absorbing the carbonic acid in the atmosphere, assimilating the carbon and giving out the oxygen almost pure. In darkness, on the contrary, plants do not give out oxygen, but carbonic acid. 2. It is also under the influence of light that the green subthe light, the paler it becomes. Humboldt slightly tinged stance of vegetables is formed. The less a plant is exposed to some garden cress with green by exposing it to the brightness of two lamps, and De Candolle obtained a deeper green by using six lamps. 3. The transition from day to night, or from night to day, gives rise to remarkable movements in the leaves and flowers of plants, phenomena popularly described as the sleep and waking of plants. The influence of light is here so apparent, that as soon as the sun appears above the horizon, the sleeping plants awake and open their leaves and petals. By exposing these plants to the artificial light of a sufficient number of lamps, De Candolle succeeded in modifying their habits. He thus saw sensitive plants expand by night, and others open their calixes by day when placed in a dark room. 4. Light also exercises a directing influence upon the branches and roots, the former seeking and the latter avoiding it, at least in many plants. It has been ascertained that all the colours of the spectrum do not exert the same degree of influence upon the branches and roots, but that between blue and violet the greatest intensity of action is produced. 5. Light imparts to vegetables a power of suction. This was demonstrated by the following experiment. Three plants of the same species and the same size were placed in three separate vases containing water, one of which was put in a dark place, the second in a mild light, and the third in the sunshine. The first was found to have imbibed very little water, the second more, and the third by far the most. DOUBLE REFRACTION. Double Refraction is a property by which many crystals present two refracted rays for each one incident on their surface, the consequence of which is, that, on looking at an object through these crystals, it appears doubled. Double refraction was first observed by Bartholin, in 1647, but |