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I offer any conjectures in reply to this question, I wish to make a brief statement of some additional observations on the subject in general.
It is well known that, cæteris paribus, the pupils are smaller when the eyes view an object placed at a near, than at a greater distance. This fact is also observed when the eyes are adapted for distinct vision at these distances respectively; and when an object at a greater or less distance, than that at which the eyes are fixed for distinct vision is glanced at, as in the preceding experiments, the conformation of the eyes remaining the same, not only is the mutual disposition and relation of the humours different, but the size of the pupil is also different from that which is proper to the eye when adapted for distinct vision at the distance of this object,being less in the former and greater in the latter case.
This circumstance may have an influence in inducing an unconnected dispersion of light, as will be further noticed hereafter. At present I wish to add another remark which may be found to throw some light on the same point. An object is seen smaller by one eye than by both; in myself by the right than by the left; and it is seen smaller when the eyes are fixed for distinct vision at a distance greater or less than that at which the object is placed. In the first case the pupil is seen to become larger; in the second, the right eye is known to be of less refractive power; and in the two last instances both the size of the pupil and the conformation of the eye are changed, the pupils being larger and the refractive power of the eyes less, in the former case, and the reverse in the latter.
From this view of the subject it would appear that a certain size of the pupils and a certain co-adaptation of the humours of the eye, are necessary for distinct vision, for securing the object viewed from a fringe of the prismatic colours, and for ensuring its due apparent magnitude. In further confirmation of this view some experiments were made with the application of the extract of belladonna to the eyes, the
general result of which I proceed to state. The sight is
rendered larger, as has been stated by other experimentalists, and it is no longer perfectly achromatic ::—a pen, for instance, cannot be mended at all, as indistinct and coloured vision is induced by bringing the object near enough to the eye to admit of its being seen distinctly in the ordinary state of the eye; but the eye is assisted by a convex lens, and a person naturally short-sighted is enabled for the time to dispense with the use of the concave glasses, and the vision becomes distinct and free from colour partly on closing one eye, but still more on viewing the object placed even still nearer, through a small perforation in a card; but the indistinctness of vision and the dispersion of light are again induced by bringing the object viewed still nearer to the eye. Distant objects are also seen indistinctly and coloured, a dark coloured object having appeared to three persons who have performed this experiment, to have been tipt and fringed with purple.
There is a certain distance at which an object is seen tolerably distinctly with both eyes, under the influence of the belladonna; at a nearer distance the object appears indistinct and coloured to both eyes, but becomes distinct and colourless when viewed by one only; at a still nearer distance it is seen indistinctly and coloured by one eye, but becomes distinct and free from colour, on interposing a card perforated with a small hole.
The convex lens and the perforated card equally prevent indistinctness and dispersion, but on different principles; the former converging, and the latter excluding, the extreme rays of the pencil of light.
I am indebted for the repetition of some of the experiments with the extract of belladonna, which deserve to be further prosecuted, to Dr. J. Davy, and Dr. A. Fyfe; to whom, after an interval of five years, I now beg leave to return my best thanks for their friendly assistance.
After this detail of experiments, it may not be wrong to conclude this Paper by the following observations and queries, as they may induce other experimenters to prosecute the subject.
It is still a matter of dispute, whether the human eye be perfectly achromatic. If any dispersion of the rays of light
in their course to the retina, in ordinary vision, do in reality occur, it is in so limited a degree as to occasion no inconveniThose physiologists, therefore, who consult the sense alone, are of opinion, that the achromacy of the eye is perfect; and it has been attempted to explain this achromacy on the principle of the construction of the achromatic telescope, which, indeed, it is supposed to have suggested. But the idea that the humours of the eye are so co-adapted, that the dispersion produced by one is corrected by a contrary dispersion occasioned by the other, is probably erroneous. In the achromatic glasses, the dispersion induced, by a convex lens, is remedied by a similar but contrary dispersion effected by a concave lens, —or, at least, the principle thus stated is secured. In the construction of the eye, however, the rays, in their course to the retina, appear only to undergo successive convergencies, at least by refraction; and consequently the degree of dispersion is also augmented successively, at each transition of the rays of light from one humour to another.
There is a part of the eye, however, the action of which has not perhaps been fully ascertained.-The iris is supposed to regulate the quantity of light admitted to the retina, and in vision at near distances, to exclude those rays which would otherwise fall with too great obliquity on the crystalline lens. But are there not other effects of the iris, not sufficiently adverted to, in the inflection and dispersion of the rays of light at the edges of this part of the eye?—and may not these effects be similar to the operation of the concave lens, in the achromatic eye-glass? A small perforation in a card induces an inflexion and a dispersion of the rays of light which pass through it; may not the finely fringed edge of the iris induce these changes in a still greater degree? Those coloured rays of light which are most refrangible, are also the most inflectible. Now when the light is intense, or when any divergent rays of light strike the eye, and the eye is so conformed as to induce great convergency of the rays by refraction, and consequently, when the dispersion of the light must be great and very obvious, the pupil is then most contracted,
and the inflective and dispersive effect of the iris, greatest. May not this effect of the iris counteract the dispersion of the rays of light induced at their refraction by the humours of the eye? And may not this operation of the iris thus insure, in ordinary vision, the achromacy of the eye?
On this supposition we should conclude that whenever the disposition of the humours of the eye, and whenever the size of the pupil, was not in just proportion mutually and relatively to the intensity and direction of the rays of light, an unconnected dispersion of the rays of light would occur, and the eye would cease to be achromatic. Is not this in effect the case, in the experiments which have been detailed in this Paper, in which a manifest uncounteracted decomposition of the rays of light actually occurred?
ART. VII. On Cryptogamous and Agamous Vegetation. From the French of C. F. BRISSEAU MIRBEL.
HE subject of cryptogamous and agamous vegetation has been purposely reserved for a separate Section. The relation between the floral organs of this part of the vegetable creation, and those of the phænogamous part, has not yet been demonstrated in a way to justify the combination of the facts which belong to the one, into the same point of view with those which belong to the other.
In cryptogamous plants, the floral organs are extremely minute, of very distinct forms from those of phænogamous ones, and are often concealed from our sight by peculiar integu
In agamous plants, either there are no floral organs at all, or else they are of a nature that has eluded the research of the naturalist up to the present hour.
In the plants of this lower degree in the scale of organization in the vegetable creation, propagation is carried on by suckers, bulbs, propagula, and seminula. The two first modes being equally appropriate to the phænogamous plants, and familiar to
every one, do not require any particular notice in this place. The two last are those to which we shall here turn our attention..
Propagula are peculiar to the agamous division. They shew themselves in the form of a powder on the surface of the plant; are at no period enclosed within a germen; and have been deemed, with great appearance of probability, mere fragments of the external texture of the vegetable. The races of entire genera are continued by these means alone.
Seminula are common to both the agamous and cryptogamous divisions. They are minute organic bodies, which reproduce the species, and possibly differ from the seeds of the phænogamous division only in the smallness of their volume. Cryptogamous seminula are evolved from germens that form a constituent part of a real pistil. Agamous seminula are developed in conceptacles, a sort of germens, which having never formed any part of genuine pistils, offer no trace of either style or stigma. These diminutive seeds lie sometimes loose in the cavity of their conceptacles, or are at others confined several together in elytra, a kind of partial conceptacles contained within a common one, which may be considered in this case as the involucre.
The term agamous is of very recent date. From the time of Camerarius, who first demonstrated the sexes of plants, down to a very late period, botanists were divided, into those who admitted the existence of sexes in no plant whatever, and into those who maintained that no species in the vegetable department of the creation was without them. Exclusive views, like these, have their rise in the tendency of the human mind, to draw general conclusions from partial facts, and which is ever the strongest in regard to those points, concerning which we know the least.
The philosophy of Linnæus was far from being untainted with prejudice, any more than that of so many others. Instead of sifting and discussing the theories of those who preceded him, he laid it down as an axiom, that the law of regeneration in vegetables was necessarily the same throughout the whole system. It was he that devised and brought into use the term cryptoga