stems are short in the extreme, and bear sharp pointed leaves on either side; at its summit it carries a calyptra cup-shaped; the oval receptacle rests on a very short stalk. The colour is yellow-green, and the plants grow in a scattered fashion, not in a dense patch, as is usual with Jungermanniæ. Its turned-in leaves procure it the name of J. calyptrifolia (No. 9). The same bank nourishes the long straggling branches and glittering foliage of the J. polyanthos. On close examination we find small stipules cut into two points beneath the stem, from behind which little rootlets spring, fastening the branch at a distance of every few lines to the clay. A few weeks later and the patch will be strewn with white calyptræ, glassy footstalks, and slender crosses (No. 5). Once more rocks rise on the side of the path, but here it is the north exposure that nourishes the Liverwort. Numerous slender branches of dark green form a network, and we are inclined to pass them by as Algæ, and irrelevant to the subject of the day; even the lens fails to discover anything more than mere inequalities in the thickness of the threads. But we carry it home, plunge it in water for a few moments, and apply the microscope; and lo! the threads are rounded stems, set on either side with closely compressed leaves, clasping with their heart-shaped bases the slender stems; it is the J. cordifolia (No. 2). Other overhanging rocks are draped with the spreading branches of the robust J. platyphylla, lying one above another tile-fashion, and affording good handfuls of foliage. The colour is olive-green, varying in hue, and the fruit is rare; but no moss is commoner on the perpendicular surface of rocks than this Liverwort. Here the leaves are divided in two lobes, like those of the J. obtusifolia, but the smaller lobe is turned to the under side of the branch; there is a stipule also, about the size of the smaller lobe, and situated under the stem, between the rows of turned-in lobes; this gives an appearance of very close and abundant foliage to the under part of the branch. Trees growing about these rocks have occasionally a very verdant patch at their roots; the strikingly fresh green attracts the observer, and he finds the plant to be J. serpyllifolia, a minute Liverwort with entire leaves placed horizontally on either side the stem, and moderately branched.

Let us search the woods and hedges immediately around us, and ere a score of moist days have been thus passed, few will be dissatisfied with their sheet of Jungermanniæ.

EXPLANATION OF PLATE XXI.

Fig. 1. Jungermannia Asplenoides.-2. J. cordifolia.-3. J. connivens.— 4. J. complanata.-5. J. polyanthos.-6. J. bidentata.-7. J. reptans. -8. J. dilatata.-9. J. calyptrifolia.-10. J. obtusifolia.-11. J. furcata.-12. J. pinguis.-13. J. epiphylla.

COLOUR-BLINDNESS.

BY JABEZ HOGG, F.L.S., &C.

HE eye-that index of the soul, that channel of human knowledge conjures up a host of feelings when the mind is directed to it, as an object of especial attention. Of the five senses with which most of us have been blest, the loss of sight seems to be the greatest calamity that can befall us. Reflect for a moment on the condition of those deprived of this exquisite gift. To what a sad state are they reduced who, in a perpetual darkness in the midst of light, have not anything like a conception of what we mean when we talk of the golden sun, the bright stars, the ever-varying tinted flowers, the beauty of spring, the glow of summer fields, the ripening fruits of autumn, and all beside that clothes the face of nature so beauteously to our eyes!

Our theme, however, is not with those who have so large a claim to our sympathies, but rather with others among us who suffer from a partial kind of blindness,-not necessarily a mechanical or optical defect, but one which is almost unknown or unrecognized by those who suffer from it, and, being ignorant of its existence themselves, cannot easily be persuaded to believe it.

An explanation of this curious defect will be worth while listening to, the more so as many eminent philosophers have suffered from it; and it is perhaps owing to this circumstance that so much time and attention has been given to the investigation of so curious an anomaly. It is well known that a ray of light, from any source, may be divided by means of a prism into a number of rays of different refrangibility, forming a series, and called a spectrum, the most familiar instance of which is the rainbow. The drops of rain falling between the sun and the eye act as so many prisms, and each ray is thereby bent or refracted to a different angle, the red most and the blue least; and as thus the rays of light are made to enter the eye separately, we have produced the beautiful prismatic phenomenon of the rainbow, the outermost colour of which is red, the innermost violet, and the intermediate, from slightly intermixing and overlapping each other, we respectively name orange, yellow, green, blue, and indigo. The three homogeneous colours-yellow, red, and blue-have been shown by

Mr. Field, in a satisfactory manner, to be in the numerical proportional power as follows:-yellow, three; red, five; and blue, eight. When these three colours are reflected from any opaque body in these proportions, white is produced; they are then said to be in an active state, but each is neutralized by the relative effect that the others have upon it. When they are absorbed, they are in a passive state, and black is the result. When transmitted through any transparent body, the effect is the same; but in the first case they are material or inherent, and in the second impalpable or transient. Colour therefore depends entirely on the reflective or refractive power of bodies, as the transmission or reflexion of sound does upon their vibratory powers. By the undulatory theory of light, philosophers account for the variously coloured rays of the solar spectrum, by calculating the differences in the frequency of the vibrations of each ray-that is, the rays of light are supposed capable of vibrating in waves of different lengths. The shortest waves produce violet light, the longest red; and with such precision have some of the more complex phenomena of light been studied, that mathematicians have absolutely been able to calculate the number of vibrations necessary to produce an impression of either white or coloured light. For instance, the periodical movements of the medium in white light regularly recur at equal intervals, five hundred millions of millions of times in a second of time; in the sensation of redness, our eyes are affected four hundred and eighty-two millions of millions of times; of yellowness, five hundred and forty-two millions of millions; of violet, seven hundred and seven millions of millions; and so on.

How seldom do the most reflecting amongst us think, as we gaze on the flowers composing a bouquet, and inhale their fragrance which perfumes the surrounding air, that in order to distinguish the yellow tint of the laburnum, five hundred and forty-two millions of millions of undulations of light must occur; that the ruby fuschia requires the eyes to receive four hundred and eighty-two millions of millions of undulations in a second; that the violet's tint is only distinguishable when seven hundred and seven millions of millions of vibrations have penetrated to the sensitive retina!

When objects are illuminated by homogeneous yellow light, the only thing which can be distinguished by the eye is the difference of intensity or brightness. It is now a generally received opinion that different bodies, according to the manner in which their minutest particles are arranged, possess the power of variously absorbing a part and reflecting the other portion of the rays of light that fall upon them; and that on the proportions of the rays absorbed and reflected does the