OPTICS. 



the retina, L M . Let the crystalline, O P, 

 be brought forward, and, C V, the dis- 

 tance of the focus from the crystaline, 

 will be increased ; but, because of the 

 great proportion that A C, the smallest 

 distance that admits distinct vision has to 

 F C, the focal length of the crystaline, 

 the distance, C G, of the crystaline 

 from the retina will be more increased 

 than C V, so that C G and C V may be- 

 come equal, and thus the focus made to 

 fall exactly on the retina. 



These powers of accommodation are 

 however limited, and the sight is said to 

 be perfect, when the eye can adapt itself 

 to any distance within the usual limits, 

 and when it cannot, vision is indistinct. 



Defective sight arises from an incapa- 

 city of altering the position of the crys- 

 taline within the usual limits. 1. When 

 it cannot be brought close enough to the 

 cornea, near objects appear indistinct ; to 

 this defect people in years are generally 

 subject. 2. Where the crystaline cannot 

 be drawn sufficiently near to the retina, 

 remote objects appear indistinct ; this is 

 the defect under which myopes, or short- 

 sighted people, labour. In each of these 

 cases, the images of the different points 

 in the object would be diffused over small 

 circles on the retina ; and so being inter- 

 mixed and confounded with each other, 

 would there form a very confused picture 

 of the object : for in the former case 

 (fig, 19), the image of any point would 

 be formed behind the retina, as the re- 

 fraction of the eye is not sufficiently 

 strong to bring the rays (diverging so 

 much as they do in proceeding from a 

 near point) to a focus at the retina. This 

 defect will therefore be remedied by a 

 convex glass, a b, which makes the point 

 whence the rays now proceed more dis- 

 tant than the object; therefore the rays 

 falling on the eye will now diverge less 

 than before, or else be parallel, and will 

 of course be brought to a nearer focus, 

 viz. at the retina. 



In the latter case the image is formed 

 before the retina (fig. 20) because the re- 

 fractive power of the eye is too great to 

 permit rays so little diverging (as they 

 do in proceeding from a distant point) to 

 reach the retina before they are collect- 

 .ed into a focus ; in this case the defect is 

 supplied by a concave glass, a b t which 

 makes the point whence the rays di- 

 verge, nearer than the object ; conse- 

 quently, the rays falling on the eye will 

 now diverge more than before, so as when 

 refracted through the humours not to 



come to their focus before they reach the 

 retina. 



Spectacles are constructed on the above 

 principles, concave for short-sighted, and 

 convex for long-sighted people. See SPEC- 

 TACLES and VISION. 



"Of microscopes and other optical in- 

 struments." The impediments to the vi- 

 sion of very near objects arise from too 

 great a divergence of the ravs in each 

 pencil incident on the eye, and are reme- 

 died by the microscope. Tnis instru- 

 ment is of two kinds : 1. refracting; and, 

 2. reflecting. 



The refracting microscope is either 

 single or compound. The former is a 

 small double convex lens, of a short focal 

 length ; the object is placed in its fo- 

 cus, by which disposition the rays of 

 each pencil emerging from the lens be- 

 come parallel, and so are brought to 

 their respective foci on the retina by 

 the humours of the eye : the magnify- 

 ing power of the instrument appears 

 from hence. 



The apparent lineal magnitude of an 

 object, seen with this instrument, is to its 

 lineal magnitude seen with the naked 

 eye, as the least distance that admits of 

 distinct vision with the naked eye, to the 

 focal length of the lens; for these magni- 

 tudes are as the angles under which the 

 object appears, i.e. inversely as the dis- 

 tances at which it is viewed. 



A compound microscope is composed 

 of two double convex glasses, the broader 

 next the eye. In this instrument the dis- 

 tance of the object from the object-glass 

 is to be made greater than the focal 

 length of that lens; then the image will 

 be formed at the focus conjugate to the 

 place of the object, and the eye-glass 

 being placed atits own focal distance from 

 the image, will make the rays emerge 

 parallel to each other, and consequent- 

 ly produce distinct vision. See MICRO- 

 SCOPE. 



To enlarge the field of the compound 

 microscope, it is usual to insert a broad 

 lens, as in the astronomical telescope, be- 

 tween the object-glass and the image. 



The reflecting microscope is thus con- 

 structed : In the extremity of a broad 

 tube insert a concave speculum N U (fig. 

 21) ; a point O in its axis, whose distance 

 from the vertex, V, is greater than the 

 focal length of the concave, is the place 

 for the object, whose image will conse- 

 quently be formed at the focus, G, con- 

 jugate to the point O : at the distance of 

 its own focal length, L G, place a double 

 convex lens, a b, by which the image will 



