38 



OPTICS. 



Parts of the scale; 



Wool of Mr. Western's South Down, some spe- 

 cimens 24.5 



Lioiuza Wool, 24 to 29, generally 2 



Paular Wool, 24 to 2i), generally 2f>.5 



Alpacca Wool, about 26 



Farina of Laurtistinns 26 



By ela lid Marino Wool 27 



Merino South Down Wool... 28 



Seed of Lycopodiinn, beautifully distinct 32 



Wool of a' South Down ewe 89 



Coarse Wool, Sussex 4 



Coarse Wool, from some worsted 00 



In order to find in parts of an English 

 inch the diameter of the particles or 

 fibres of any of these bodies, we have 

 only to multiply sooW Dv P 16 number in 

 the table. Thus, Blood being 7, we have 

 the diameter of its particles ^oijoo x 7 

 = TTOOO> or 4T8? th P art of an inch - 

 CHAPTER XVI. Colours and Struc- 

 ture of Mother of Pearl Colours of 

 the grooved surface of the Lenses of 

 Animals Mr. Barton's Iris orna- 

 ments. 



A VERY interesting class of colours, 

 which have been recently employed in 

 the useful arts, are exhibited by polish- 

 ed surfaces, either of glass or metal, 

 when they are crossed by parallel 

 grooves very near to each other. As 

 these colours have been long ago ob- 

 served in mother of pearl, we shall 

 begin by giving an account of the very 

 remarkable phenomena which this sub- 

 stance exhibits. 



Mother of pearl has in general a very 

 irregular surface, and therefore in order 

 to observe its properties, we must select 

 a piece which is regularly formed, and 

 which in general has an uniform white 

 colour in daylight. When this piece 

 of mother of pearl has been ground 

 on both sides upon aflat stone, or upon 

 a piece of metal with very fine emery, 

 it is ready for use. If we now place our 

 eye near one of its surfaces, so as to 

 see the image of a candle in it by re- 

 flexion, we shall observe a reddish dull 

 image S, (fig. 39.), free from any of 

 Fig. 39. 



the prismatic colours. This image is 

 formed by the ordinary laws of reflexion, 

 and its dullness arises from the rough- 

 ness of the surface in which it is seen. 



On one side, suppose the left of this 

 dull image, there will be seen a brighter 

 image A, which is a real prismatic 

 spectrum of the candle containing the 

 same colours, and dispersed nearly as 

 much as in the spectra formed by a 

 large refracting angle of flint glass, 

 but having its blue extremity b nearest 

 the ordinary image. The distance of 

 the red part r of this image from the 

 ordinary image, is, in a specimen now 

 before us, 7 22'. On the outside of 

 this coloured image, and nearly at the 

 same distance beyond it on the one side 

 as the dull image is on the other, will 

 be seen a mass of light, C, of a crimson 

 colour, which becomes green by varying 

 the inclination of the plate. These three 

 images, S, A, C, are all in the same 

 straight line. 



If we now polish the surface of the 

 mother of pearl, the ordinary dull 

 image S will become brighter, and a 

 second prismatic image B will start up 

 directly opposite to ttie first, and at the 

 same distance from the common image 

 S. This second prismatic image has 

 exactly the same properties as the first 

 one, with the exception of having its 

 brightness a little diminished by the 

 polishing, and of its never being accom- 

 panied with the mass of crimson light. 

 When the polish of the surface is re- 

 moved by grinding, the second prisma- 

 tic image B disappears, and the first A 

 is restored to its former lustre. By re- 

 peating the preceding experiments on 

 the opposite surface of the mother of 

 pearl, the same phenomena are ob- 

 served ; t\\e first prismatic image A, and 

 the mass of coloured light S being now 

 on the right hand of the ordinary 

 image, and the second prismatic image 

 B on the left hand of it. 



If when the plate of mother of pearl 

 is sufficiently thin we examine the 

 transmitted light, we shall see the same 

 appearances which we see by reflexion ; 

 the image which is brightest by reflexion 

 being faintest by transmission. In this 

 case, too, the blue end b of the prismatic 

 image is nearest the ordinary image, 

 and the red end r farthest from it. 



The communication of the colours of 

 mother of pearl to other substances 

 upon which it is impressed, which v, as 

 first pointed out by Dr. Brewster, 

 forms, to those who see it for the first 

 time, one of the most surprising pheno- 

 mena in optics, while to the scientific 

 observer it furnishes the true cause of 

 the origin of the colours. If we take 



