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VOLUME THE SIXTH.
READING furnishes the mind only with materials of knowledge; it is thinking makes what we read ours. We are of the
ruminating kind, and it is not enough to cram ourselves with a great load of collections ; unless we chew them over
JOHN CASSELL, LA BELLE SAUVAGE YARD, LUDGATE HILL.
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, uniforin 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 CICLOPEDIA ; comprising a vast amount of solid and useful information in a popular form, and at a price unprecedented cren in the present age of Cheap Literature. The higliest encomiuins have been bestowed upon our labours by a large portion o! the Public Press, by learned Profess rs, 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 tlic seasonable and valuable aid we have rendered, and of deep regret that we have brought our labours to a closc. We take our respectful leave of on friends, thanking them sincerely for the assurances they have given us that we have not incoured in rain.
XXVIII. La vierge aux ruines. Sections I., II., III., IV.,
V., with exercises, etc.
L'emploi du temps, with exercises, etc.
roi et la jeune fille ; Sections I., II., with ex.
charité; Section I., with exercises etc. ....
I. II., with exercises, etc......
XXVI. Problems in Adfected Quadratic Equations ;
XXXIV. Inierest; Compound Interest..
XVII. David Hume
III. Whether we always do right by obeying the XXVII. The Lyre : Edmund Burke; Truth ..... 711
XXIX. Excellence of the Holy Scriptures : Crescentius;
Rectitude of Character; Address to the
Ocean; the Bible; the Downfall of Poland;
the Love of Truth
sidered essential to Free Agency; Whether
Whether Morality belongs to Principles as
VI. Relative Pronouns; Interrogative Pronouns 519
IX. Whether Virtue and Vice belong only to
XIII. Reflective Verbs: Passive Verbs
Plants; Double Refraction; Polarisation 409
XX. Of the Passive Verb, the Regimen of Verbs,
trial Magnetism; the Compass
XXI. Idiomatic Use of certain Verbs; of the Adverb
LVII. Electricity; Measure of Electric Forces
LXIX Dynamical Electricity; Phenomena of Induc-
Irradiation is a phenomenon in which white objects, or those ON PHYSICS, OR NATURAL PHILOSOPHY. of a bright colour, when seen on a dark ground, appear larger
than they really are. The reverse takes place with a black No. LIII.
object on a white ground. It is thought that irradiation arises
from the circumstance that the impression on the retina extends (Continued from page 396.)
more or less beyond the outline of the image. The effect of
irradiation upon the apparent magnitude of the stars is very THE EYE CONSIDERED AS AN OPTICAL perceptible, and they may thus appear several times larger INSTRUMENT.
ihan they really are.
According to the researches of M. Plateau, irradiation varies In sensible part of the Retina.—The retina is not equally sensi- considerably in different persons, and even in the same person tive in every part, as is proved by the following experiment of on different days. This philosopher has also shown that Mariotte. Let two black spots be made on white paper, at a irradiation increases with the Lrightness of the object and the distance of from half an inch to an inch from each other. Then, length of time it is observed. It is perceptible at all distances, when the paper is brought very near the eye, let the right eye is increased by divergent lenses and diminished by convergent be fixed upon the left spot without preventing it from seeing ones. the other. If the paper be now slowly withdrawn, the right
Accidental Halo. Contrast of Colours.—Colours which instead spot will disappear for a time, but reappear soon afterwards if of following the impression of an object like accidental colours, the paper is still further removed. The same thing will hap- appear round the object itself when attentively looked at for pen if the right spot is looked at with the left eye. Mariotte some time, are called accidental halos. The impression of the has remarked that at the moment when the spot ceases to be halo is the reverse of that of the object; that is to say, if the visible, its image is projected upon the insertion of the optic object is distinct, the halo is obscure, and vice versa. nerve in the interior and lower part of the eye. This insensible Contrast of colours is a reciprocal action which takes place part of the eye is called punctum cæcum, or the blind point. between two colours nearly allied, and by virtue of which
Continuance of the Impression on the Retina.-On whirling each of them assumes the complementary colour of the other. round a lighted coal with rapidity, we perceive a sort of band This contrast was observed by M. Chevreul, who profoundly of continuous fire. Similarly, the rain which falls in drops, investigated the subject, with a view to ascertain the laws of appears like liquid threads in the air. These appearances are the phenomenon. It is attributable to the reciprocal action of owing to the fact that the impression produced by objects the accidental halos above mentioned. M. Chevreul found on the retina remains after the object is removed or replaced that on red and orange being placed side by side, the red by another. The duration of this continuance varies accord- inclined to violet and the orange to yellow. If the experiment ing to the sensibility of the retina and the intensity of the be made upon red and blue, the red inclines to yellow and the light. M. Plateau of Brussels has discovered, by various blue to green. With yellow and blue, the yellow passes into methods, that it is on the average about half a second. orange and the blue to indigo, and similarly with many other
The impressions of colours as well as forms remain after the combinations. It is needless to remark how important is the removal of the objects that produce them, for if we divide a bearing of this subject upon the manufacture of cloth, carpets circle into sectors and paint them different colours, on turning and other coloured articles. Those who would wish to be it round, the colours mix and produce the sensation of the successful in combining colours must understand the principles colour which would be formed by their mixture. Thus blue of the effect of contrast. and green produce the sensation of green; yellow and red The Eye not Achromatic.-It was long the custom of philo. that of orange, blue and red that of violet; and the seven sophers to attribute to the human eye the property of perfect colours of the spectrum that of white, as is shown by Newton's achromatism, but this notion cannot be admitted without disc. There are several curious apparatus, the effects of which qualification after the various experiments of Wollaston, are explained by the continuance of the impressions upon the Young, Fraunhofer and Muller. Fraunhofer observed that in retina. Such are the thaumatrope, the phenakisticope, the a telescope with two glasses a very fine thread placed inside kaleidophone, and Farraday's wheel.
the instrument is distinctly seen through the eye-piece when Accidental Images.-If a coloured object be placed upon a the telescope is illuminated with red light only, but ceases to dark ground and looked at attentively for some time, the eye be visible, if, without altering the position of the eye-piece, soon becomes wearied and the intensity of the colour grows the telescope is illuminated with light of a violet colour. To feeble. On directing the cyes to a white piece of paper or on see the thread again, it is necessary to diminish the distance the ground, we perceive an image of the same form as the between the two glasses, much more than is required by the object, but of a complementary colour; that is to say, a colour refrangibility of violet light. Hence it is evident that part which would form white if it were combined with that of the of the effect is due to the aberration caused by the refrangiobject. In the case of a green object, the image is red, and bility of the eye. rice versa; if the object is yellow, the image is violet. These Muller found that, on looking with a single eye at a white disc coloured appearances were remarked by Buffon, who gave on a black ground, the image is clear when the eye is adapted them the name of accidental images or colours. Accidental to the distance of the disc, that is to say, when the image is colours continue for a length of time, proportioned to that formed on the retina. But he observed, that if the eye is not during which the object was observea, and to the intensity of adapted to this distance, that is to say, if the image is formed the light upon it. Generally speaking, they do not disappear at a distance either in front or at the back of the retina, the gradually and without interruption, but present alternate disc appears to be surrounded with a very narrow blue band. disappearances and reappearances. It is well known also that He concluded from this and other experiments that the eye is if, after having looked attentively at a coloured object, we achromatic as long as the image is received from the focal close the eyes rapidly, and as firmly as possible, so as to distance, or as long as the eye is adapted to the distance of exclude the light, and even screen them from the light by the object. It is not yet known what is the precise cause of means of a thick piece of cloth over them, the accidental images this apparent achromatism of the eye, but it is generally
attributed to the delicacy of the pencils of light which pass Various theories have been proposed to account for the through the aperture of the pupil, and to the fact that the rays phenomenon of accidental colours. That of Darwin is deser- being of various refrangibility, and meeting the media of the ving of mention. He thinks that the part of the retina wbich eye almost perpendicularly, are very little refracted, and hence is wearied by one colour, becomes insensible to the rays of the dispersion is not perceptible. As to spherical aberration, that colour, and is only capable of impressions of the com- we have already seen how that is corrected by the iris, plementary colour ; also, that this part of the retina spon- which is a real 'partition, arresting the marginal rays that taneously assumes an opposite mode of action, which produces have a tendency to go beyond the crystalline, and only sufferthe sensation of the complementary colour. The first part of ing those to pass which are nearest the axis. this theory does not explain the appearance of accidental Short Sighi and Long Sight. The usual cause of short-sightcolours even in darkness, and the second part is merely a edness is a too great convexity of the cornea or crystalline. statement of the phenomenon of accidental images.
The eye being then too convergent, the focus instead of being VOL. Y.