formed chiefly by the leaves and petals, though also in a less degree by the stems and roots, like the respiration of animals, is attended with and marked by the conversion of oxygen into carbonic acid: it is the respiration of plants.

Again, vegetables at certain times and under certain circumstances decompose carbonic acid, and renovate the atmosphere by the restoration of its oxygen; but this occasional restoration is dependent not on the respiratory, but the digestive system; it in part arises from the decomposition of water, but chiefly from the decomposition of carbonic acid, absorbed either in the form of gas or in combination with water, either by the roots or leaves, or both: and here, again, the analogy holds good between the functions of respiration and digestion in animals and plants, for to both is carbonic acid deleterious when breathed, and to both is it invigorating to the digestive system when absorbed as food.

The presence or absence of light seems to have little or no influence on the respiration of vegetables; but it produces very notable effects on their assimilating powers, by enabling the specific vitality of the plant to separate from air and water those principles which they hold in solution or combination, which are necessary for their support, and to liberate such others as may be too abundant in the crude aliment they introsuscept; for plants growing in the dark become etiolated, and assimilate but little solid matter, and scarcely ever form their appropriate and peculiar secretions: thus assimilation tends to increase, respiration to decrease the solid materials of the plant. Another agent of great power, though hitherto scarcely noticed, is the electricity generated by plants, and excited by their profuse perspiration. I have frequently found, by experiments such as have been related, the perspiration of leaves and plants to be a moiety of their own weight, and this for days and weeks together and in some extreme cases, as was shewn by Hales, plants will perspire during twenty-four hours as much water as is equal in weight to two-thirds of their gross bulk. This abundant exhalation must be designed to answer some important end, and this use I believe to be (as at another time I shall attempt to shew) to keep the plant constantly in that electrical state which will favour the entrance of fluid into

the roots, and its passage upwards through the stem into the leaves it also must have much influence in changing and overcoming the chemical attractions of the elements of vegetable food, and seems to be one great agent in the curious process of vegetable assimilation.

CONTRIBUTIONS TO THE PHYSIOLOGY OF VISION.

No. I.

UNDER the above title it is proposed to bring forward those stores of knowledge on this subject which have been hitherto locked up in the repositories of foreign scientific literature. The physiology of vision has a peculiar claim on the attention of philosophers, as presenting some of those links which connect physical with mental phenomena. Metaphysicians, physiologists, natural philosophers, and artists, have equally made it an object of their study; and the names of Baptista Porta, Leonardo da Vinci, Kepler, Descartes, Newton, Berkeley, Reid, Buffon, Darwin, Wells, Brown, Young, &c., are among those who have advanced the inquiry by their investigations and discoveries. That the subject is of such equal interest to so many different classes of inquirers, is perhaps the cause that, as a whole, it is so imperfectly known. Each person who occupies himself with its study, looking at it only from his own point of view, disregards those facts which he considers as belonging to the province of others, and thus is unable to arrive at those general conclusions which can only be obtained from a complete survey of all the various phenomena and their relations. To render some assistance towards forming a more complete theory of vision, 'we shall successively give an account of the discoveries of Purkinje, Goethe*, Mile, Müller, Plateau, &c. The number of these interesting memoirs on this interesting branch of science, which have been entirely unnoticed in this country,

* An account of the Farbenlehre,' or theory of colours, of this illustrious poet and philosopher, will form one of the subsequent papers of this series.

might surprise us, did we not know that the same neglect extends to many other important departments of knowledge.

'Beiträge zur Kenntniss des Sehens in subjectiver Hinsicht, etc.' (Essay on the Subjective Phenomena of Vision, by Dr. J. Purkinje, Professor of Physiology at the University of Breslau.) Prague. 1823.

This little volume has excited considerable interest in Germany; it relates to those appearances which, independently of external objects, are perceived in the organ of sensation itself. To distinguish these phenomena from those which arise on the presence of their appropriate external objects, the author employs the term subjective, which, as denoting this class of phenomena better than any other we are acquainted with, and, to avoid circumlocution, we have purposely retained; it will, however, on consideration, be perceived, that the term is not strictly proper, as, correctly speaking, all phenomena, as such, are subjective, i. e. in the mind; and were we, without qualification, to admit the classification of phenomena into objective and subjective, we should be unable to determine, with any degree of accuracy, where the objective ends or the subjective begins. Thus, the vessels of the eye and the retina itself are subjective, considered as parts of the visual organ; yet we shall see that in some of Dr. Purkinje's experiments they become real objects, and are perceived as such. But we shall not further discuss this question; what we have said will be sufficient to explain the term subjective as employed by Dr. Purkinje and by ourselves in the following extracts. We now proceed to an abridged description of the most interesting of Dr. Purkinje's experiments.

I. Luminous Figures produced by rapid alternations of Light and Shade.-These figures are most distinctly seen in the following manner: the eye-lids being closed and the eyes directed towards the sun, the observer quickly moves his hand with the fingers spread, from one side to the other, so that the luminous rays are alternately intercepted and admitted; at the beginning of the experiment a yellowish-red glare is perceived which is afterwards replaced by a beautiful and regular figure, which it is, however, impossible to fix or determine,

unless the experiment be continued for some length of time, Figs. 1, 2, 3 and 4 represent the phenomena as observed by Dr. Purkinje in his right eye.

Fig. 1 consists of small squares, chequered as in a chess board, and alternately bright and dark; this entire figure is bounded by zigzag lines, which are continually varying in direction, length, and brightness, and which appear rather more illuminated than the squares; at the centre of the square field is a dark point, with a luminous area, surrounded by rapidly moving semicircular lines, which nearly resemble rose-leaves in form; these are around and principally below the luminous area; below these semicircular lines is a field of hexagons, the circumferences of which are gray, and the centres white. This figure, Dr. Purkinje says, may be obtained very distinctly, and without any admixture of the other figures, if the experi ment be modified so that the eye, being open, is directed towards an equally illuminated white wall, and the spread fingers are moved before it; if the experiment be made as before described, the secondary figures rather predominate. The appearances also take place under various other circumstances and modifications; for instance, the semicircular lines at the centre of the figure are particularly visible when the eyes are directed as near as possible to the flickering flame of a candle. The square field is also seen by looking at Newton's circle of colours, when it is in rapid motion; in this case it is not necessary that the colours be distributed in any particular manner, for the experiments will succeed, if the segments of the disc be merely alternately light and dark; the nearer the segments are to each other, the less rapid the motion of the circle is required to be, but bright sunshine is indispensable to the experiment. Lastly, the figure is seen when a wheel rapidly revolves between the eye and the sun, or a strong light; and it appears accordingly that the general condition of the phenomenon is a rapid alternation from light to shade.

The secondary figures, as Dr. Purkinje calls them, are indistinct when the experiment is made whilst the eyes are open; they appear under two modifications, the rectangular spiral, and the star with eight rays: at the commencement of the experiment, whilst the eyes are not over excited, both figures

appear, as it were, mingling with each other, the radiated figure evidently predominating (fig. 2); but as the experiment continues, the rectangular spiral (fig. 3) becomes more visible, and the star gradually disappears: the central line of the spiral is the smallest and darkest, as will be seen by the figure, and has an oblique direction to the right and below; the line itself consists of a darker axis and a bright margin, and is divided, as it were, into joints; towards the periphery of the figure the axis becomes enlarged, and fades to a greyish tint; the lateral margin also loses its brightness, and at the termination of the spiral line the illumination seems even to be inverted —that is, the centre appears light, and the circumference dark : it is, however, impossible to determine with great accuracy the external parts of the figure; the intervals between the coils of the spiral are occupied by a faint gleam of the squares of fig. 1.

In the two oblique lines of the star (fig. 4), the light axes are brighter than in the other two lines; in the latter, on the contrary, the dark borders are of a deeper black. The spiral and radiated figures are in continual motion and fluctuation: sometimes the rectangular spiral changes into a triangular one; at other times the centre of the star dissolves, and the rays intersect each other at various points, or become parallel, or form squares, triangles, &c. The four figures above described are, however, those which most frequently occur; and though, as Dr. Purkinje judiciously remarks, these subjective phenomena might appear to other eyes different from what he observed, yet the experiments made by others at his request seem to confirm his own observations: we may therefore, perhaps, be justified in concluding that the above phenomena do not depend on a morbid or individual condition, but physiologically result from the very organization of the human eye.

The figures in Dr. Purkinje's left eye, the sight of which was very weak, were very indistinct, but did not in any other respect appear different from those perceived by the right eye: the squares were more like network formed by curved lines; the secondary figures were apparently the same as before, but, as might be supposed, were in the opposite direction.

II. Figures produced by pressure on the eyeball.—If gentle