172 



EDINBURGH JOURNAL OF NATURAL HISTORY. 



mine which several iiavigatora have traversed arise from the same cause. Sir Hum- 

 phrey Davy states that in Switzerland, when the tint of the lakes passes from blue to 

 green, it is because the waters are impregnated with vegetable substances. Lastly, 

 near the mouths of large rivers, the sea has often a brownish colour, arising from the 

 mud and other earthy matters held in suspension. It is necessary, ihen, to insist on 

 the colours produced by foreign matter mixed with the water, so that they may not 

 be confounded with those to be described. 



The sky-blue tint of the sea is modified, and sometimes even entirely changed, in places 

 where the water is not very deep. This is because the light rellected by the bottom 

 reaches the eye, mixed with the natural light of the water. The effect of this super- 

 position may be calculated by the laws of optics ; but we must add to our acquaintance 

 with the nature of the two commingled tints, what is more difficult to be ascertained, 

 that of their comparative intensities. Thus, a bottom of yellow sand reflecting but 

 lightly, gives the sea a green tint, because yellow mixed with blue, as is well known, 

 produces green. Now, without changing the shades, if you replace the dull yellow 

 by a bright yellow, the slight blue of pure water will scarcely produce a lively light 

 green, and the sea will appear yelloio. In the Bay of Loango, the waters are always 

 deep red, insomuch that they are said to be mixed with blood ; and Captain Tuckey 

 ascertained that the bottom is intensely red. Let us substitute for this bright red 

 bottom one of the same colour, but dull or reflecting slightly, and the waters will then 

 appear orange, or even perhaps yelhw. 



An objection, which at first sight appears impoi-tant, is made to this mode of re- 

 garding the subiect : — a bottom of white sand, it is said, ought not to alter the tint 

 of the sea, for if white weakens the colours with which it mixes, it yet does not change 

 the hue. But there is a ready answer to this objection : for how can we be certain 

 that the sand at the bottom is white ? Is it not in the open day, after we have brought 

 up a portion, and exposed it to the white light of the sun or the clouds ? Is the sand 

 in the same condition when beneath the water? If in the open air you were to il- 

 luminate it with red, green, or blue light, it would appear red, green, or blue. We 

 have then still to inquire what colour strikes it at the bottom of the water. 



Water is in the condition of many other bodies which philosophers have very deeply 

 studied, and which possess two kinds of colours ; a certain colour which is transmitted., 

 and another, quite different, which is refiected, "Water appears of a blue colour by 

 reflection, and some imagine it to be green by transmission. Thus, water disperses 

 in all directions, after having blued it, a portion of the white light which went to il- 

 luminate it. This dispersed light constitutes the proper colour of liquids. As to 

 the other irrcgidarhj transmitted rays, their passage through the water makes them 

 green, and this the more intensely the thicker the traversed mass. 



This being admitted, we may return to the case of a not very deep sea, with a 

 bottom of white sand. This sand receives the light only through a stratum of water. 

 The light then is green when it strikes the bottom, and it is with this tint that it is 

 refiected ; and in the second traject which the luminous rays make through the same 

 liquid in returning from the sand to the open air, this green tint sometimes so pre- 

 dominates as to prevail over the blue. This, then, perhaps may be the whole secret, 

 which is to the practical navigator, in the time of calm, a certain and invaluable index 

 of great depths. We say in tune of cahuy for when the ocean is agitated, the waves 

 suitably elevated may in fact convey to the eye so large a quantity of transmitted or 

 green rays, that the reflected blue rays' shall be entirely masked. 



Let us imagine a triangular prism placed in the open air horizontally before an ob* 

 server, somewhat lower than he is. This prism cannot by refraction conduct to tho 

 eye any ray coming directly from the atmosphere. On the contrary, the anterior 

 face of the prism will throw towards the observer a reflected atmospheric pencil, a 

 great nart of which, it is true, would pass over his head. This portion would require 

 to be bent in its course, to be refracted from above downwards to reach the eye. A 

 second prism, placed like the first, but nearer to the observer, would produce this 

 effect. 



From these few words of explanation, the reader has already, no doubt, made the 

 assimilation which must lead to the conclusion towards which we are tending. The 

 waves of the ocean are a kind of prism ; no wave is ever solitary ; the continuous 

 waves advance nearly in parallel directions. When two waves, then, approach a 

 vessel, a portion of the light which the anterior face of the second wave reflects tra. 

 vej'ses the first, is there refracted from above downwards, and thus arrives at the ob- 

 server placed on the deck. Again, then, we see transmitted light, light consequently 

 made green, reach the eye at the same time with the common blue tints ; but theso 

 are the phenomena of great depths over white sand produced without deep water, and 

 the green colour of the sea arises from the predominance of the transmitted over the 

 reflected colour. 



We have now hastily traced the imperfect outlines of a theory of the colours 

 of the ocean, that voyagers may thereby be directed in the investigations which 

 they may have occasion to make on the subject. The examination of circum- 

 stances which may oppose this theory will suggest to them experiments, or at least 

 observations, which otherwise they probably would not have thought of. Thus, every 

 one will understand that the prism waves ought not to produce the same effects under 

 differences in the direction of their propagation, and some variation in the colour of 

 the sea will be expected under a change of wind. This phenomenon is apparent on 

 the Swiss lakes. Is it so also on the open ocean? 



Some persons persist in assigning an important influence to the blue of the sky in 

 the production of that of the sea. It appears to us that this idea may be subjected 

 to a decisive proof, thus. The blue rays of the atmosphere do not return from the 

 water to the eye till alter they have been legularly reflected. If the angle of refrac- 

 tion equals 37^, they are polarised. A piece of tourmaline will then completely eli- 

 minate them, and thus the blue of the sea will be seen without any extraneous mix- 

 ture. 



In order to get rid, as much as possible, of the influence of reflection, when exam- 

 ining the colours of the ocean, some able navigators have recommended that we should 

 always examine it through the aperture of the ship's rudder. In this way the water 



in some points of view exhibits beautiful violet tints ; but we may easily satisfy'our- 

 selves that these tints are merely the effect of contrast, and that they proceed from 

 atmospheric light feebly reflected in an almost perpendicular direction, and coloured 

 by their approximation to the transmitted green colours which almost invariably sur- 

 round the rudder. 



To those who may wish to develope this attempt to explain the colours of the sea, 

 or to those who may desire to refute it, and substitute a better, it will be necessary 

 to begin with investigating the colour of the water when seen by transmission WiM Z/« 

 aid of diffused light. Those who will recall to their mind the pre-eminently green 

 hue of the cut edge of a crystal glass, even when the latter is only illuminated in fronk 

 and perpendicularly, will be aware of the importance of this remark. The following 

 appears a very simple means of obtaining a satisfactory conclusion. 



I shall suppose the observer to be furnished with one of those large hollow prisms 

 which philosophers are in the habit of using when they study the refraction of liquids. 

 To render our apprehensions more precise, let us make the refracting angle 45**, 

 We shall then suppose that the prism is partially immersed in water, so that the edge 

 of its refracting angle is downwards, and that one of its faces, that which is farthest 

 from the observer, shall be vertical ,* whence it will result as a necessary consequence, 

 that the other face will be inclined to the horizon at an angle of 45°. Under these 

 circumstances, the light which moves horizontally in the water at a fraction of an 

 inch below the surface, that which constitutes its edge colour, if the expression may 

 be used, will strike perpendicularly the vertical side of the prism ; it will penetrate into 

 the interior of the instrument, traverse the small quantity of air which it encloses, 

 reach the second plate, and then be reflected vertically upwards. In looking 

 upon this inclined surface, the observer may then judge of the proper colour which 

 the water has by refraction, quite as well as if his eye were in the liquid. In this 

 form the experiment is so simple and easy, and requires so little time, that we shall 

 venture to request the Academy to recommend to our voyagers to repeat it as often 

 as possible, not only in sea-water, but also in that of lakes and rivers. When science 

 shall be enriched with all these observations, we shall no longer run the risk of con- 

 structing theories s toner or later to be contradicted by facts. — Comptes Rendus. 



MISCELLANEOUS. 



Toes of the African Ostrich It had been alleged some time ago, that Dr 



Riley of Bristol had discovered a rudimentary toe in the Ostrich, in addition to the 

 two which it requires no minute investigation to discover ; but Mr Thomas AUis of 

 York, having carefully examined a specimen for the express purpose of satisfying 

 himself on this point, asserts, " that whatever rudimentary toe may have been dis- 

 covered in Dr Riley's Ostrich, there was certainly no third toe" in the one examined 

 by him. He further corrects a mis-statement of Cuvier regarding the number of 

 phalanges in the toes of the Ostrich, having found them to be four and five, in place 

 of four and four. 



Blikdworm. — Mr Rylands, in a letter to the Editor of the Naturalist, mentions 

 his having captured, on Woolston Moss, a specimen of Anguis agreeing, in almost every 

 respect, with Pennant's description of A. Eryx. Its description is as follows : — 

 Length, 14f inches ; belly of a bluish lead-colour, with a few scattered white spots-, 

 which become more continuous and regular under the tail ; the remainder of the body is 

 greyish-brown, with three longitudinal dark lines, one extending from the head along 

 the back (becoming indistinct towards its termination) to the point of the tail, the 

 others broader, and extendingthe entire length of the sides; scales, &c., as in .<4. /ra- 

 gilis. In conclusion, he remarks that further observations being necessary to settle 

 the question, he is desirous of obtaining such. 



There is in my collection a specimen of the Bllndworm, found in Aberdeenshire, 

 and which I kept alive for some_time. Its tail had been partially broken off 

 and cicatrized, so that its length cannot be ascertained; but what remains is 10 

 inches long, and the lost part must have been three or four more. The common 

 Blindworm is described as being greyish -brown above, bluish-black beneath, with seve- 

 ral parallel rows of small dark spots along the back, and a dusky band on each side ; 

 and the variety or supposed species named Eryx is said by Pennant to have the belly 

 of a bluish iead-colour, marked with small white spots irregularly disposed ; the rest 

 of the body greyish-brown ; with three longitudinal dusky lines, one extending from 

 the head along the back to the point of the tail, the others broader, and extending 

 the whole length of the sides. My specimen, which was' found on a moor in Buchan, 

 agrees sufflciently with this alleged Anguis Eryx, and yet I have reason to be- 

 lieve is nothing but the common Blindworm. It is of a light greyish-brown abovo, 

 with a central undulated narrow line, and on either side three still more slender 

 lines of black ; then on each side is a broader black line or band, below which are five 

 interrupted or dotted lines of black ; and th© belly or lower surface is bluish-black in 

 its whole length, but with six paler lines, and numerous whitish spots beneath the 

 tail. Such variations cannot be considered as specific. Sometimes the Slow-worm 

 has the upper parts without lines at all, sometimes with a single line, and sometimes 

 with seven lines; the lower parts either uniformly bluish-black, or with a broad band 

 of the same colour as the back, and on each side a central dark band. The number of 

 lines is, I think, fourteen on each side, twenty-eight in all, that being the number of 

 si'ries of scales. Some specimens are destitute of white spots on tVie lower part of the 

 abdomen and tail, but in most individuals they are more or less apparent. Young, 

 or at least small individuals, generally have the lines more distinct. ^ 



Edinbuhch: Published for the Proprietor, at the Office, No, 13, Hill Streei. 

 London: Smith, Elder, and Co., 65, Cornhill, Glasgow and the West of 

 Scotland: John Smith and Son; and John Macleod. Dublin: George 

 YooHG. Paris ; J. B. Baillikre, Ruede I'Ecole de Medecine, No. 13 bis. 



" ^ THE BDINBURGH TRINTING COMPANY. 



