fixed stars was subjected to the same examination; in some of these, lines were observed in positions different from those before observed. The electric light was tried in the same way; the points of two conductors were connected by a fine fibre of glass, along which the succession of sparks was so rapid as to produce the appearance of a fine line of light. In the spectrum formed by this light, (without passing any aperture,) lines different from any of the former were observed. The light of several flames was similarly examined, and several curious results obtained.

Such is a brief outline of the most important parts of M. Frauenhofer's experiments; they indicate a very remarkable property of light, and present appearances which we believe have not yet been accounted for on any known principles. We must here take occasion to remind our readers, that the discovery of the fact itself, (though evidently unknown to M. Frauenhofer,) was made some years ago by Dr. Wollaston. His experiment was however somewhat different; and owing to the great superiority of his glass, M. Frauenhofer has the merit of having ascertained the almost infinite number of those lines, which in Dr. Wollaston's experiments appeared only a few. M. Frauenhofer must also have the credit of being the first to apply these lines to the purpose of accurate determination of the dispersive power, although Dr. Wollaston made a few observations of this kind. It may be satisfactory to many of our readers if we here mention, that with an ordinary prism of English glass, the principal lines may be very well seen, by looking through the prism at a narrow aperture in a shutter, or screen placed against a window so as to receive the light of the clouds; this was Dr. Wollaston's method: his experiments are given in the "Phil. Trans." 1802; he examined also the light from flame. If any of our readers are inclined to try the experiment, we recommend particularly to them to look at the blue part of a candle flame through a narrow slit; the separation of the colours is very wide and complete.

The mere inspection of the prismatic colours is sufficient to show that the different parts of the spectrum, independently of their colour, possess very different degrees of brightness or illuminating intensity. The late Sir W. Herschel was, we believe, the first who attempted any accurate determination of these relative intensities; he found the greatest illumination in the yellowish-green space, and a gradual decrease from thence towards each extremity. M. Frauenhofer tried similar experiments by a different method, and his determinations were made with greater attention to exactness than perhaps any former; but there appear to us two essential difficulties in his method.

In the first place, the intensity of each coloured ray was to be

equalized with the white or yellowish light reflected by a plane mirror from a lamp; M. Frauenhofer considers it easy, with a little practice, for the eye to judge of this equalization with the requisite accuracy. This we must confess appears to us very doubtful; though the sensation of colour and of intensity may possibly depend on modifications of the same cause, yet the two sensations follow such very different laws, and that difference is dependent upon principles so wholly unknown to us, that we can hardly conceive the possibility of abstracting so entirely from the idea of colour that of intensity, as to enable the mind to decide in any thing like a certain and satisfactory manner, upon the equality of illuminating effect in lights of two different colours simultaneously presented to the eye.

Another and more serious difficulty appears to us to arise from the following considerations: Supposing the illuminating intensities to be really equal; it is well established that if two rays of light, one of a colour approaching more to whiteness than the other, be presented in juxta-position to the eye, the deeper colour of the one will be diluted by the proximity of the lighter colour of the other; that is to say, though not actually combined or blended together, the sensation which the one produces in the eye tends to diminish that which arises from the other. If this, as is highly probable, is owing to the different convergency required for the two, it will obviously take place in a greater degree in proportion as the coloured ray differs in refrangibility from the white. (See Edin. Phil. Journ. No. 19, p. 33.)

Whatever weight may be attributed to the objections against this particular method, it is certain that the illuminating intensity sustains a regular decrease from the central yellowish green to the violet on one side, and the red on the other. The series of numbers given by M. Frauenhofer decrease in a more rapid ratio than those found by any other observers, and the tendency of the causes just considered as influencing his results, would be precisely that of producing this rapid diminution. But the decrease of illuminating power towards the red boundary, will become a point of considerable interest in the sequel.

M. Frauenhofer's observations on the illuminating powers of the prismatic rays, led him to several suggestions of practical importance in the construction of telescopes. He attends particularly to the distinction between diminishing the aberration of colour, and producing greater distinctness in the image; as also to the aberration from the want of achromatism in the human eye. When different specimens of glass were examined by the accurate test of the spectral lines, the difference in their dispersive powers was shown, when not otherwise capable of detection. M. Frauen

hofer found differences of this kind in specimens taken not only from the same crucible, but from the opposite parts of the same piece of glass. By unwearied diligence and laborious trials he has, however, at length succeeded in the manufacture of flint glass, to such a degree, that in a crucible containing four hundred pounds, two pieces, one taken from the bottom, and the other from the top of the same mass, exhibited absolutely the same power.

This becomes the place for noticing the results obtained by a fellow labourer in the same work, M. Guinand. The small publication we have named relative to this individual, is one which we have perused with considerable interest, as exhibiting a remarkable instance of the power of intuitive mechanical 'skill, in surmounting all the obstacles which circumstances and situation conspired to place in the way of its developement. M. Guinand was the son of a joiner at Neufchatel: as a youth he worked at that trade; subsequently made watch cases; and thus acquiring some idea of casting metals, undertook, on examining a reflecting telescope, to make one; in which he soon succeeded, without any knowledge of optics, and left entirely to his own resources for every part of the work. His next attempt was to make a pair of spectacles. He learnt the art of grinding and_polishing the lenses by having once witnessed the process. He hence proceeded to make lenses for telescopes, and constructed several small refracting ones. He now accidentally became acquainted with the principle of the achromatic object glass: and all his energies and labours seemed concentrated upon the means of endeavouring to procure glass free from imperfections for this purpose. This is in fact one of the most difficult problems with which the practical optician is concerned; and the patience, the sagacity, the perseverance, which M. Guinand displayed, in a long series of attempts under the most discouraging circumstances, to obtain his object, were truly surprising. At every failure he seemed to be occupied solely in studying the cause which had occasioned it. And thus, step by step, he contrived to approach at length towards the wished for object, and produced glass more free from striæ and imperfections than any before made. Every disappointment taught him some further improvement, and it was thus that he acquired, what is perhaps the distinguishing characteristic of his method, the mode of joining together into one large disk separate pieces of glass, selected as the most perfectly homogeneous. These he contrived to soften and unite together again, after which they were formed into the required lens, without any perceptible joining or imperfection; in this way he has formed lenses of twelve or eighteen inches diameter. In 1805, his fame had reached M. Frauenhofer, who invited him to Bavaria, to give his important services to the

establishment of Benedictbauern, where glass for optical purposes is largely manufactured under M. Frauenhofer's direction. The glass made by M. Guinand has since become known over Europe; specimens have been tried by the opticians and astronomers of France and our own country. The report of that eminent artist, M.Tulley, as to its great superiority to any made in this country, is couched in the strongest terms; and there can be little doubt that owing to the very perfect transparency which it possesses, we may expect a great increase in the power of refracting telescopes, hitherto so much limited in their degree of improvement. M. Guinand returned to his native place, and continued the construction of telescopes with uncommon ingenuity and success, himself not only having melted, formed, and polished the glasses, and calculated the adjustments, but also constructed every part of the apparatus, and put it together. This remarkable example of untaught genius died in 1823, aged seventy-six. His secret is confided to his son, who undertakes to continue the manufacture so important to the scientific world, upon the same principles as his father.

We before mentioned that M. Frauenhofer's first attempts were directed to obtaining homogeneous light by means of flames and coloured media; in this he was unsuccessful. Dr. Brewster, however, and M. Herschel have been more fortunate. In the memoirs above named by these two distinguished individuals, a great number of experiments are detailed, having in many instances a similar object in view.

Dr. Brewster was in want of homogeneous light, to illuminate objects under microscopic examination; Mr. Herschel wished to obtain it for the prosecution of certain optical researches. Dr. Brewster after numerous trials ascertained the remarkable fact, that almost all bodies in which the combustion is imperfect, such as paper, linen, &c. gave a light in which strictly homogeneous yellow rays predominated; that the yellow light increased with the humidity of these bodies; and that a great proportion of the same light was generated when various flames were urged mechanically with a blow-pipe, or a pair of bellows. He thence concludes, that the yellow rays are the produce of an imperfect combustion. However, the most important circumstance was, that the presence of aqueous vapour increased the quantity of yellow light; this was a new fact, and supplied Dr. Brewster with a lamp whose light was truly homogeneous. Diluted alcohol is the pabulum he employs, and he has suggested a convenient form for a lamp for the purpose wanted.

Various media, such as coloured glasses, were also tried. Dr. Brewster investigated the effect of heat in changing the tints of

these glasses; in some the power of absorbing particular colours is altered transiently, in others permanently. He tried the effect of different media in absorbing the different rays of the spectrum, and has given delineations of the spectrum as seen through different coloured glasses.

In Mr. Herschel's experiments the object was nearly the same in the first instance, but he has pursued it in a somewhat different manner from Dr. Brewster, and has arrived at some other results of considerable consequence.

He first examined, as also Dr. Brewster did, the effect of certain coloured glasses in almost obliterating certain coloured spaces in the spectrum, whilst others were transmitted in all their brilliancy. This fact was first noticed by Dr. Young: Mr. Herschel, in applying to the examination of it the uncommon powers of his analytical skill, has resolved the phenomena into their most general expression, and thus traced the cause of many interesting consequences which otherwise would not have been deduced.

For example: one of the glasses he tried was of a ruby red colour; this permitted to pass almost the whole red, and a considerable portion of the orange; and even in strong lights a portion of yellow or a trace of green, but the rest were obliterated. He represents the effect by conceiving a straight line divided according to the proportions of the coloured spaces, to be taken as the abscissa, and at each point ordinates erected representing the proportion of rays transmitted by any medium; the extremities of these ordinates give a curve, which he calls the type of this medium. The nature of this curve is determined by observation for each medium; but Mr.Herschel has given an analytical expression, showing the law by which the nature of the curve is altered, according to an increase of thickness in the medium: this is in fact one of the most curious parts of the subject.

"It would appear at first sight," Mr. Herschel observes, " that the effect of doubling or tripling the thickness of any coloured medium, would simply be to increase the depth and intensity of the tint, but not to alter its character. If a white object appear blue through a blue glass, we should expect it to appear still bluer through two, and yet more so through three such glasses. The above formula shows, however, that this is so far from being the case, that the tint of the emergent pencil is essentially dependent on the thickness of the medium; and that it is only from a knowledge of the relative values of the ratios of the intensity, after traversing a thickness equal to unity, for the various parts of the spectrum, that we can say à priori, whether the tint of a thick glass will retain any similarity to that of a thin one of the same kind." (p. 447.)

The fact is, the quantity of any coloured ray, transmitted by an