SCIENCE. 



beams to a 

 point op- 

 vposite to 

 ahe sun. 



Science, rays parallel, diverging and convcrging.~-1f the radiant 



Curiosities point or object from which rays diverge is a little with- 



" in the anterior principal focus of a spherical lens, the 



'"""""**'" * central parts of the lens will render the diverging rays 



slightly divergent, the part of the lens without this -will 



render them parallel, and the outer rim will cause 



them to converge to a positive focus behind the glass. 



The same effect may be .produced by a reflecting 



mirror. 



11. On the Convergence of ike Solar Beams to a point 

 opposite to the Sun. 



On the eon- The divergency of the solar beams, when the sun is 

 vergency of descending in the west, is a phenomenon which oc- 

 the solar curs so frequently, that the most careless observer must 

 have had occasion to notice it. This phenomenon, 

 however, is sometimes accompanied with one of an 

 opposite kind, viz. the convergency of the solar beams to 

 a point opposite to the sun, and as far below the horizon 

 as the sun is above it. This phenomenon is extremely 

 rare ; and we are not aware that it has been described 

 more than once, viz. by Dr. Robert Smith of Cam- 

 bridge, who observed, that he once saw it upon Lin- 

 colnheath. He describes it as " an apparent converg- 

 ence, of long whitish beams, towards a point diame- 

 trically opposite to the sun. For as near as I could 

 estimate, it was situated as much below the horizon 

 as the sun was then elevated above the opposite point 

 of it." " In the unusual phenomenon," Dr. Smith af- 

 terwards adds, " I well remember, that the converg- 

 ing sun-beams towards the point below the horizon, 

 were not quite so bright and shining as those usually 

 are which diverge from him, and that the sky beyond 

 them appeared very black, which certainly contributed 

 to the evidence of this appearance." Smith's Optics, 

 vol. ii. Remarks, p. 57, 58. 



On Saturday, the 9th October, 1824, Dr. Brewster 

 : had the pleasure to observe this curious phenomenon 

 \vhen travelling from Melrose to Edinburgh, and of 

 pointing it out to two friends who accompanied him. 

 It was first seen at that part of the road opposite to 

 the avenue to Kirkhill, the seat of John Tod, Esq. at 

 about a quarter past four o'clock. The sun was then 

 *considerabiy elevated above the Pentland range of hills, 

 -and was throwing out his diverging beams in great 

 beauty through the interstices of the broken masses of 

 clouds which floated in the west. The eastern part 

 of the horizon, where the converging lines were seen, 

 was occupied with a dark black cloud, as described by 

 Dr. Smith, and which seems necessary as a ground for 

 rendering visible such faint radiations. The converg- 

 ing beams were much fainter than the diverging ones, 

 and the point to which they converged was as near as 

 could be estimated, as far below the horizon as the 

 sun was above it. About ten minutes after the phe- 

 nomenon was first seen, the convergent lines mere 

 Mack or very dark. This aro^e from the real beams 

 having become broad, and of irregular intensity, so 

 that the eye took up, as it were, the spaces between 

 the beams more readily than the beams themselves. 



In order to explain the cause of this phenomenon 

 minutely,, several diagrams would be necessary, for 

 which we cannot at present find room ; but we think 

 it may be perhaps more easily .understood from the 

 following illustration 



Let us suppose a line to join the eye of the observer 

 and the sun ; let rays issue from the sun in all possible 

 directions, and let us suppose that planes pass through 

 these radiations, and through the line joining the obser- 



ver and the sun, which will be their comtnon intersec- 

 tion, like the axis of an orange, or the axis of the earth, 

 through which there passes all the septa of the former, 

 and all the planes passing through the meridians of 

 the latter. An eye, therefore, skuated in that line or 

 common intersection of all the planes, will see them 

 diverging from the sun on cue side, and converging 

 towards the opposite point, just as an eye in the axis 

 of a globe would perceive all the planes passing 

 through the meridians, diverging on one side and con- 

 verging on another. See Dr. Brewster'* Journal of 

 Science, Vol. II. p. 136. 



12. Improvements on ilie Magic Lantern. 

 In our article OPTICS, Vol. XV. p. 63Q, we have de- 

 scribed the magic lantern in its general form. A very 

 considerable improvement, however, has lately been 

 introduced into the construction of the sliders on which 

 the objects are painted, by which very curious trans- 

 formations are produced. In one of these the eyes are 

 left empty in the picture of a fine old head, and by 

 moving another slider containing a pair of eyesj from 

 one side to another, the head appears to turn as if it 

 were alive. In another a smith is seen with his ham- 

 mer resting on the anvil, and by repeatedly pushing 

 forward a subsidiary slider and drawing it back, the 

 smith raises his arm and strikes the iron, from* which 

 sparks immediately issue. In the same way various 

 changes in the attitude, the dress, and the action of fi- 

 gures on the sliders may be introduced into the picture, 

 which give a great variety, and a new interest to the 

 formerly tame representations of the magic lantern. 



1 3. Singular optical illusion seen through a telescope. 



If we direct a telescope to the surface of a distant 

 field on which there are no objects, such as trees, 

 houses, &c. and if the field of the telescope contains 

 nothing but the surface of the field, the eye will speed- 

 ily recognize that the field is horizontal or slightly in- 

 clined to the horizon, from the perspective of the fur- 

 rows or drills upon its surface, or even from its aerial 

 perspective, provided the difference in the distances of 

 the nearer and the remoter end is considerable, and the 

 air sufficiently hazy. 



The field, however, may be so situated, and have 

 such an inclination, that when seen through the tele- 

 scope it appears like a perpendicular or vertical wall 

 of earth. This phenomenon we have often seen in di- 

 recting a telescope to a field above Melrose Abbey on 

 the northern acclivity of the north-west Eildon Hill. 

 This field is capable of being ploughed in the direction 

 of its greatest declivity; but when it is viewed through 

 a telescope, the slope is such that the furrows do not 

 appear to converge, and the eye cannot readily per- 

 ceive any difference between the breadth of the furrows 

 at the remote end of the field, and their breadth at 

 the near end. The observer, therefore, immediately 

 concludes that the field must be nearly a vertical plain 

 rising in front of him. This deception is a very re- 

 markable one, and produces a singular effect on the 

 mind when the field is covered with a crop and when 

 crows, &c. light upon it. I have not yet observed the 

 effect produced when it is in the act of being ploughed. 

 It is very probable that the impossibility of ploughing 

 a vertical plain may remove the deception upon the 

 principles which we have already explained in another 

 part of this article. See Dr. Brewster's Journal qf 

 'Science, No. VII. 



14. Optical deception of Le Cat. 

 M. Le Cat has described a curious optical deception 



Improve- 

 merits on 

 the magic 



lantern. 



Singular 

 optical il- 

 lusion seen 

 throug^ a 

 telescope. 



