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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
if their readers would observe and imitate them: all the rest at best are but particulars fit to be turned into knowledge;
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 is 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 friends, thanking them sincerely for the assurances they have given us that we have not laboured in vain.
LESSONS IN READING AND ELOCUTION.
Classical Learning: Dame Nature's Charms
Moral Judgment; Belief in God, as con-
VII. Moral Habits; Nature of Virtue
XXX. The Scholar's Mission; the Treasure that
XIII. Reflective Verbs: Passive Verbs
ON PHYSICS, OR NATURAL PHILOSOPHY.
(Continued from page 396.)
THE EYE CONSIDERED AS AN OPTICAL
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.
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 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 which 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 colours even in darkness, and the second part is merely a statement of the phenomenon of accidental images.
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