G2 



OPTICS. 



intensely black, and yet may, perhaps, 

 refract it within themselves so long, un- 

 til it happen to be stifled and lost, by 

 which means they will appear black in 

 all positions of the eye without any 

 transparency. 



Having thus stated, as clearly as we 

 can in such a small compass, Newton's 

 celebrated theory of the colours of na- 

 tural bodies, we shall lay before the 

 reader several illustrations and con- 

 firmations of it drawn from the disco- 

 veries of modern science, together with 

 such facts as may be of use either in 

 modifying or extending the views of our 

 great philosopher. 



1. One of the most curious facts of 

 this description was discovered by M. 

 Thenard. Having obtained some very 

 pure phosphorus by repeated distilla- 

 tions, he melted it in hot water, when 

 it became of a whitish yellow colour as 

 usual. When it was allowed to cool 

 slowly, and become solid, it preserved 

 this colour unchanged, and was semi- 

 transparent ; but when it was thrown 

 in its melted state into cold water, it 

 became suddenly opaque and absolutely 

 black. Its nature, however, remained the 

 same, for when it was melted again it 

 became yellow and transparent as before. 

 In repeating this experiment, M. Biot ob- 

 served a very curious fact. When the 

 melted phosphorus was thrown into cold 

 water, some little globules of it remained 

 yellow and liquid, but the instant 

 they were touched with the end of a 

 piece of glass tube they became solid 

 and absolutely black. As the same 

 piece of phosphorus can thus be made 

 opaque or transparent at pleasure, it 

 affords a fine example of the influence 

 of the arrangement of the particles, and 

 of a change in their size in producing 

 the opposite conditions of yellowish 

 white and black. 



2. That remarkable substance called 

 tabasheer, which is a siliceous concre- 

 tion found in the joints of the bamboo, 

 exhibits some curious phenomena rela- 

 tive to its colour and its porosity. The 

 pure, varieties which have their index 

 of refraction so low as 1.1114 between 

 water and air, reflect a delicate azure 

 colour, and transmit a sort of straw 

 yellow colour. When a small drop of 

 water is put upon it, the wetted spot 

 l.vcomes instantly milk white and 

 <] aqua, although the water is absorbed 

 and enters its pores ; but when more water 

 is added so as to fill its pores, it re- 

 covers its transparency, ceases to reflect 



the azure tint, and transmits a yellow 

 less intense than before. The cause of 

 this singular property has been ex- 

 plained in the following manner by Dr. 

 Brewster, who first observed it. Let 

 -. 55, 



Fig. 55. 



be a plate of tabasheer, and A B C D 

 one of its pores highly magnified. We 

 know that this pore 'is filled with air, 

 and that when a ray of light M N 

 enters the separating surface A B at E, 

 and quits it at H, it suffers so little 

 refraction, and is therefore so little 

 scattered, that the tabasheer appears 

 transparent, and allows us to see ob- 

 jects through it distinctly. This co- 

 existence of a high degree of trans- 

 parency with a high degree of porosity 

 is unexampled in material bodies, and 

 arises from the slight difference between 

 the refractive power of air arid taba- 

 sheer. Let us now suppose that a 

 small quantity of water is introduced 

 into the pore AB C D, so as not to fill 

 it, but merely to line its circumference 

 with a film contained between A B C D 

 and abed. Then the light which was 

 formerly scattered by the slight refrac- 

 tion at E and H, in passing from taba- 

 sheer into air, will now be a little less 

 scattered at these points, since it passes 

 from tabasheer into water, where the 

 difference of refractive power is less ; 

 but while the light passes from the film 

 of water into the air at F, and enius 

 the water again at G, the scattering of 

 the rays will be very considerable, 

 owing to the great difference of refrac- 

 tive power between air and water. In 

 passing through every pore therefore 

 the light is refracted, and consequently 

 scattered no less than four times, and 

 hence the piece of tabashew when 

 slightly wetted with water must appear 



