Imagini ale paginilor
PDF
ePub

important defect. These qualities, however, are possessed by the water colours of the present day; the greater part of them are very permanent, and the others, which are not so, are capable of being prepared fresh and fresh; the number of colours moreover is great, the combinations that may be formed of them almost endless; and gradations of each may be made, from nearly white to next to black. Not only must a scale of them be had in possession, and frequently referred to, but it must be made and remade by the observer, as a mode of impressing the colours on his memory; and, unless he can carry thein in his mind, he need not attempt the chromatic observation of stars; for, as he cannot see the star and his scale of colour at the same instant, and side by side, the estimate of the star depends entirely on the memory.

Many persons may think that a mere glance at colours is enough to impress them at once on the memory, and that, without any practice at that sort of remembrance, they can keep any tint in their mind for any length of time; but a more erroneous idea was never entertained. To these unhappy persons greens are greens, and blues, blues; and they have never entered even the region of colours, and a whole world of intellectual enjoyment is for ever closed against them. Bring them to the proof of their boasted powers; shew them any portion of a landscape; and then place colours before them, and make them put down the various tints from memory, and this a week or two after the scene was witnessed. If hardy enough to attempt the task, every one of their tints will be found in error, and they will only put down one, where nature had fifty. Even the painters confess that, though colour may be a low branch of their art, yet it is the most difficult : only look at the walls of the Royal Academy, and see how rarely is a good colourist to be met with ; and, when he is, how the initiated will gloat over the matchless and magic variety and mellowness of tints, while the uninitiated can see barely more than one; and that, to them, not noticeably different from the world of common-places beside it. Only look, too, at the characteristics of those painters who draw from nature, but do not colour also from her ;-—who make their sketches in the open air with pencil or sepia, and fancy colouring to be so simple and so easily remembered, that they may do that afterwards comfortably and at home. Their works are known wherever they are seen, by the poverty of tints, and by the uniformly monotonous colours that are always employed in the same manner : the human mind cannot invent to any extent, but only put together in a novel manner materials collected from the external world. And such materials in colouring, can only be impressed on the memory by actual painstaking and laborious copying and working from nature, by making the tints and applying them in imitation of her. By such training, this branch of memory may be strengthened as well as any other ; for we find that the works of artists who adopt this method are always superior in their colouring to those of others, even when they paint from the memory or the imagination. And one of the best colourists that we have ever had in landscape painting was so impressed with the importance of cultivating the memory in this manner, that he used, even in the days of his prosperity and the high prices of his works, to spend much time in the open air making studies in oil; and then, as soon as they were made, tore them up; so that, as the followers of Cortez saw the necessity of conquering, when their commander burnt the ships in which they might have made an inglorious retreat, and exerted themselves accordingly,—in the same way, not being able to refer, when painting a picture at home, to the sketch made in the open air, he felt himself necessarily obliged to tax his powers of memory, and make them exert themselves to the very utmost. .

The second, or instrumental method of determining colour, need not be entered upon at much length here, as amateurs are not very likely to practise it; and would be working generally at a great disadvantage compared with any instrument in a public observatory specially devoted to this object. Brightness is everything with this method, and this must be commanded, both by elevating the telescope into a high region of the atmosphere, and by adopting the largest possible size of aperture; for, not only must photometrical determinations of the brightness of different sections of the spectra of stars formed by prisms, be made; but the black lines in the spectra of each star must also be carefully examined into, as all the transcendant accuracy of modern optics depends on them. This instrumental method of reducing colour to brightness and place, in addition to the exactness of the numerical determination of which it is then capable, would further overcome a most important source of error, and one which has not been touched upon in all that has gone before, and would affect to its fullest extent the method of the “senses,” namely, chromatic personal equation.

In the first volume of my Cycle of Celestial Objects, pages 302 and 303, I instanced the peculiarity of certain eyes in their being unable to distinguish colours correctly, and yet capable of proper action in every other use of them. Everyone knows those violent cases of it where a person cannot distinguish green and red and other such egregious contrasts, and would not admit such a person's observations of colour at all; but it is by no means so generally known as it should be that a personal equation of greater or less amount exists in every case, and the reason of the faulty colouring of so many artists is, that they really are not aware of many of the refinements of colour: their eyes not perceiving them, their fingers cannot render them. In one of the most intense examples, however, of this chromatic personal equation, although the person could not distinguish so bright a scarlet as the flower of the

pomegranate from the genuine green of its leaf, I have had abundant proof that his eye was able to perceive brightness, independent of colour, as acutely, if not much more so, than the generality of men.

These, then, being the advantages of the instrumental method, we may hope that they will not be lost sight of. If it be true that the Government is about to send a large reflector to Australia to observe the southern nebulæ, it might also forward another to a tropical region for observing the planets and to make chromatic observations of the stars. The Australian telescope will have more work than sufficient with the nebulæ, and the planets with their faint satellites will be low down in the north there, while we have them low in the south here; but the equatorial telescope will have them in its zenith; and it may be elevated on some table-land there far higher into the atmosphere than the Australian one can be; a very important matter where colours, rather than brightness, are concerned; for a want of the latter may be corrected merely by using a larger aperture; but a distortion of the former, once introduced, is utterly irremediable.

N 7. THE STORY OF Y VIRGINIS.

In the brief mention which I made of double stars, at pages 284-6, it ought, perhaps, to have been noted, that by the measurement of the angle which the meridian makes with a line passing through both components of the object under observation, together with measures of the distance of the stars from each other, the form and nature of the orbit are determined : and of those compound systems which have, to an almost conclusive conviction, been proved to obey the power of gravitation, none has attracted greater attention from the astrometers of the day than y Virginis.

A detailed history of this very remarkable binary-star, one of the first recognised as a revolving system, was published in my Cycle of Celestial Objects in 1814. But, as the present volume will be before readers who have not seen that work, and in order to bring up the whole matter together to the present time, I shall commence this section with an entire reprint of my former account :

CCCCLVI.

Y VIRGINIS.

AR 12h 33m 33s
DEC. S 0° 34':3

PREC. + 39.07

S 19''.84

DISTANCE

(62)

(#3)

POSITION 770.9 (- 6)

71°.4 (205) 630.6 (207) 62.7 (108) 480.8 (66) 450.5 (25) 150.0 (205)

1":6

1".2 not taken

11.3 11.0 01.8 0%.5

(12)

EPOCH 1831.38

1832.40 1833.23 1833-44 1834.20 1834:39 1835.40

(# 2)

(22)

(1)

[blocks in formation]

A fine binary star, in Virgo's right side, heretofore known as Porrima and Postvarta by Calendar savans. A 4, silvery white; B 4, pale yellow, but though marked by Piazzi of equal magnitude with A, it has certainly less brilliancy; and the colours are not always of the same intensity, but whether owing to atmospherical or other causes remains undecided. They are followed by a minute star nearly on the parallel, and about 90' off. With b, d, and », it formed the

n, XIIIth Lunar Mansion, and was designated, from its position in the figure, Záwiyah-al-'auwà, the corner of the barkers. This most instructive star bears north-west of Spica, and is 15o distant, in the direction between Regulus and y Leonis, which are already aligned. A very sensible proper motion in space has been detected in A, and there can be no doubt of B’s standing on in the same course; the most rigorous comparisons of recent observations afford the following values :

[merged small][merged small][merged small][merged small][merged small][merged small][ocr errors]

It was with much gratification that I watched this very interesting physical object through a considerable portion of its superb ellipse, and I was fortunate enough to attack it during the most critical period of its march. It is rather singular that, brilliant as these two stars are, various occultations of y Virginis by the Moon have been recorded, without allusion to its being double. So lately as the 20th March, 1780, the phenomenon was watched by nine astronomers; yet at Paris

* This was the most puzzling of ail my double-star trials; for, so unexpected was the phenomenon, that I gazed long and intently before pronouncing it round in the month of January: and it was only on repeated scrutiny that I had an impression that the object was in rather an elongated form in April, which impression was confirmed by the 21st of May. The weights were, however, added to the angle rather to substantiate my own conviction by the senses, than to attest accuracy of measure. About this time I received a letter from Dr. Robinson, of Armagh, informing me that he had no difficulty in elongating y with Sir James South's large refractor.

« ÎnapoiContinuați »