OPTICS. 



7. Converging and diverging beams. 

 When the sun is descending in the 

 \vest, through masses of open clouds, 

 the divergency of his beams, rendered 

 visible by their passage through nume- 

 rous openings, forms frequently a very 

 beautiful phenomenon. It is sometimes 

 accompanied, however, with one of an 

 opposite kind, viz. the convergence <f 

 beams to a point in the eastern horizon 

 opposite to the sun, and as far beneath 

 the horizon as the sun is above it, as if 

 another sun, throwing out divergent 

 beams, were about to rise in the east. 

 This phenomenon is rarely seen in per- 

 fection. Dr. Smith, who observes that 

 he once saw this phenomenon on Lin- 

 coln heath, describes it as ' an apparent 

 convergence of long whitish beams 

 towards a point diametrically opposite 

 to the sun, and, as nearly as he could 

 estimate, as much below the horizon 

 as the sun was then elevated above the 

 opposite point of it.' 



On the 9th of October, 1824, \ve had 

 the satisfaction of seeing this curious 

 appearance in unusual splendour. The 

 sun was considerably elevated, 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 portion 

 of the horizon, where the converging 

 lines were seen, was occupied with a 

 black cloud, which seems necessary as 

 a ground for rendering visible, by its 

 contrast, such feeble radiations. The 

 converging beams were very much 

 fainter than the diverging ones, and 

 their point of convergence was as far 

 below the horizon as the sun was above 

 it. About ten minutes after the pheno- 

 menon was first seen, the convergent 

 lines were black, or very dark. This 

 arose from the real beams having be- 

 come broad, and of unequal 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 this phenomenon, 

 which is a case of perspective, let us 



. * This disposition of the eye is a very curious 

 one, and has, we believe, never "been observed. When 

 wj look steadily at a carpet having figures of one 

 colour, green for example, upon agrmind of another 

 colour, suppose red, we shall, sometimes, see the whole 

 of the green pattern, as if the red one were oblite- 

 rated ; and, at other times, we shall see the whole of 

 the red pattern, as if the green one were obliterated. 

 The former effect takes place when the eye is stea- 

 dily tixed on the green p;irt, and the latter, when it 

 is steadily used on the red portion. It is easy to 

 conceive ihat when the retina i.> in a state of irrita- 

 tion or excitation with red light, it will more easily 

 take up, as it were, the Tision e.'a red object than of 

 any other. 



supposa a line to join the eye of the ob- 

 server and the sun. Let beams issuse 

 from the sun in all possible directions, 

 and let us suppose that planes pass 

 through these beams, and through the 

 line joining the eye of the observer and 

 the sun, which will be their common 

 intersection, like the axis of an orange, 

 or the axis of the earth, through which 

 there pass all the septa of the former, 

 and all the planes passing through 

 the meridians of the latter. An 

 eye, therefore, situated in this lin?, or 

 common intersection of all the planes, 

 will, when looking at a concave sky, 

 apparently spherical, see them diverg- 

 ing from the sun on one side, and con- 

 verging towards the opposite point, 

 just as an eye in the axis of a large 

 globe w r ould perceive all the planes 

 passing through the meridians diverg- 

 ing on one side, and converging on 

 another.* 



CHAPTER XIX. Partial fieftezion of 

 Light Absorption of Light Light 

 reflected at different angles from 

 Water Glass Metals White 

 opaque Bodies -from both surfaces 

 of Glass from a number of Glass 

 Plates. 



FROM the phenomena described in the 

 preceding chapters, the reader must have 

 observed, that when light falls upon the 

 most transparent bodies, such as water, 

 glass, &c. a certain portion of it is re- 

 flected from their surfaces. When we 

 measure the quantity reflected and the 

 quantity transmitted, we invariably find 

 that the sum of these quantities is less 

 than the light which falls upon the 

 body. Hence it follows, and the fact is 

 a very important one to remember, that 

 light is always lost in passing through 

 the most transparent bodies. This light 

 is lost in two ways : a portion of it is ab- 

 sorbed or stopped by the body and forms 

 heat, and another portion is scattered 

 in all directions by irregular reflexion. 

 When light falls on metallic bodies, 

 such as 'polished silver, or speculum 

 metal, about one half of it is reflected, 

 and the other half lost. The part lost 

 consists, as in the former case, of two 

 portions ; one of which, and by far the 

 largest, being absorbed, and the other 

 scattered by irregular reflexion. 



No complete set of experiments has 

 yet been made from which the laws of 



t See Smith's Optics, vol. ii. Remarks, p. 57, 53 ; 

 and Edinburgh Jivrnal of Science ,'So. iii. p. i&j. 



