OPTICS. 



eel images, the line joining which is al- 

 ways perpendicular to a sort of veined 

 structure in the shell,which goes through 

 its substance, and cannot be impressed 

 upon wax. These images are seen by 

 light transmitted through a thin and 

 highly polished plate of mother of pearl, 

 cut parallel to the natural surface of the 

 shell, and reduced by grinding to a thick- 

 ness of between T ,th and T^dth of an 

 inch. The nebulous images are large, 

 and their different parts have the follow- 

 ing distances from the ordinary image. 



The Extreme Red Ray 10 29' 



The Mean Ray 6 59 



The Extreme Violet ; 6 16 



By measuring with a high magnifying 

 power the width of the veins which pro- 

 duced these images, Mr. Herschel found 

 that there were 7700 of them in an inch. 

 In examining the laminae or plate 

 which composed the crystalline lens of 

 a cod newly killed, Dr. Brewster dis- 

 covered, that their surfaces gave the 

 same colour as mother of pearl, with 

 this difference only, that two and some- 

 times three prismatic images were seen 

 on each side of the ordinary image. 

 These laminae communicated their co- 

 lour to wax ; and by measuring the 

 distances of the coloured images at 

 different parts of the laminae, he 

 found that the fibres diminished gra- 

 dually from the equator to the pole of 

 the lens, tapering like needles, so as to 

 allow them to pack together into a 

 spherical superficies as they converge 

 to the pole. He discovered also in the 

 lens of the Boneto, the flying fish, the 

 herring, &c. another set of coloured 

 images exactly at right angles to them, 

 and about 16* distant from the ordinary 

 image. These images prove the exist- 

 ence of grooves or divisions across the 

 fibres, whose distance is at least the 

 1 1 OOOdth of an inch. When we consider 

 that the crystalline lens of a small fish 

 is thus composed of several millions of 

 separate fibres, and each of them sub- 

 divided into several thousand portions, 

 we cannot fail to be struck with the ex- 

 quisite beauty of such a combination, 

 and with the admirable skill by which 

 it is fitted for performing the several 

 operations of vision. 



The principle of the production of co- 

 lour by grooved surfaces has been ele- 

 gantly and ingeniously applied by John 

 Barton, Esq., of the Royal Mint, to the 

 manufacture of what he appropriately 

 calls Iris ornaments, from their reflect- 

 ing the brilliant hues of the rainbow. 



By cutting with a delicate engine paral- 

 lel grooves upon steel, at the distance 

 of from the 2000dth to the lO.OOOdth of 

 an inch, he has manufactured imitative 

 jewels and other articles of female orna- 

 ments of transcendent beauty ; and by- 

 stamping these grooves on brass with 

 steel dies, he has manufactured buttons 

 at a moderate expense. In day light, 

 these colours are scarcely distinguish- 

 able, excepting,when the surface reflects 

 the margin of a dark object upon a 

 light ground ; but in strong light, and 

 particularly in that of the sun and of 

 gas flames, the colours shine with ex- 

 traordinary brilliancy, and the play of 

 tints, which flit about with every lumi- 

 nous image, are rivalled only by the 

 matchless hues of the diamond. 



M. Fraunhofer, who has studied the 

 phenomena produced by grooved sur- 

 faces, constructed a machine by which' 

 he could draw lines in which there are 

 32,000 in a Parisian inch. 



CHAPTER XVII. Description of the 

 Eye Dimensions of the Eye For- 

 mation of Images on the Retina 

 Direction of Visible Objects Cause 

 of Erect Vision Distinct and Indis- 

 tinct Vision in the same Object In- 

 distinctness of Vision at the base of 

 the Optic Nerve Intermission in 

 the Vision of Objects seen Obliquely 

 Insensibility of the Eye to Faint 

 Light Seat of Vision Duration of 

 Impressions on the Retina Thau- 

 mat rope Single Vision with Two 

 Eyes Squinting Accommodation 

 of the Eye to different Distances 

 Longsightedness Shortsightedness 

 Ocular Spectra Accidental Co- 

 lours Colours from- the Unequal 

 Action of Light on the Eye. 

 IN the application of the principles of 

 optics to the explanation of natural phe- 

 nomena, the structure of the eye, and the 

 manner in which it performs the func- 

 tions of vision, claim particular notice. 



The human eye, of which we have 

 given a front view in (Jig. 41.), and 



Fig. 41. 



