46 



OPTICS. 



that, the external muscles which con- 

 verge the axes, produce also the ad- 

 justment by pressure on the eyeball. 

 This, however, is not the case ; as we 

 have succeeded in producing, by the in- 

 voluntary stimulus of light, an adjust- 

 ment to one distance when the axes 

 were converged to a more remote point. 



1 2 . Cause of longsightedn ess . When 

 the eye loses the power of accommo- 

 dating itself to near objects, the person 

 is said to be longsighted. This change, 

 which generally shows itself by the dif- 

 ficulty of reading small type with can- 

 dle-light, commonly takes place at the 

 age of forty, and arises from a mecha- 

 nical change in the state of the crystal- 

 line lens, by which its density and re- 

 fractive power are altered. When 

 every other part of our frame begins to 

 shrink and decay, the eye partakes in 

 the general change. The variation of 

 density takes place most frequently at 

 a particular point in the margin of the 

 lens, and requires some time to com- 

 plete its circle. At its commencement 

 vision is considerably injured, but when 

 the change has become symmetrical 

 round the margin of the lens, the sym- 

 metrical action of a convex lens enables 

 the eye to see as distinctly as before, by 

 converging upon the retina rays flowing 

 from near objects, which the unassisted 

 eye refracted in such a manner that they 

 would meet at points behind the retina. 



13. Cause ofshortsighted?iess.'When 

 the eye is not able to see distant objects, 

 and requires to bring minute objects 

 very near it in order to be distinctly 

 seen, the person is said to be short- 

 sighted. In this case, the rays from 

 distant objects are converged to points 

 before they fall upon the retina, and the 

 evil is removed by using a concave lens, 

 which delays their convergency. This 

 imperfection of the eye often appears in 

 early life, and seems* to arise from an 

 increase of density in the central parts 

 of the crystalline lens. 



14. Ocular spectra. Accidental co- 

 lours. One of the most curious affec- 

 tions of the eye, is that in virtue of 

 which it sees what are called ocular 

 spectra, or accidental colours. If we 

 place a red wafer on a sheet of white 

 paper, and, closing one eye, keep the 

 other directed for some time to the cen- 

 tre of the wafer, then if we turn the 

 same eye to another part of the paper 

 we shall see a green wafer, the colour 

 of which will grow fainter and fainter 

 as we continue to look at it. This 



green image of the wafer is called an 

 ocular spectrum, or the accidental, or 

 opposite, colour of red. By using dif- 

 ferently coloured wafers, we obtain the 

 following results : 



Colour of the Wafer. Colour of the Spectra. 



Black White. 



White Black. 



Red Bluish Green. 



Orange Blue. 



Yellow Indigo. 



Green Violet with a little Red. 



Blue Orange Red. 



Indigo.. Orange Yellow. 



Violet Bluish Green. 



If we arrange all the colours in a circle 

 in the proportions given in Chap. viii. 

 p. 24, the red and the violet extremities 

 of the spectrum meeting at 0, then the 

 accidental colour of any other colour 

 will be always found directly opposite 

 that other colour, and for this reason 

 these colours have been called opposite 

 colours. 



The same thing maybe done more 

 easily in the common spectrum A B, 

 (fig. 46.) Take half the spectrum A B, 



A 



Fig .46. 



Violet. 



Indigo. 



Orange. 

 Red. 



viz. A m, or B m, in a pair of com- 

 passes, and having set one foot in the 

 colour whose accidental colour is re- 

 quired, the other foot will fall upon the 

 accidental colour. This law of acci- 

 dental colours, derived from observa- 

 tion, maybe thus expressed : The acci- 

 dental colour of any primitive colour, 

 is that colour which in the prismatic 

 spectrum is distant from the primitive 

 colour half the length of the spectrum. 

 If we suppose the primitive colour to 

 be reduced to the same degree of inten- 

 sity as the accidental colour, then we 

 shall find that the one is the comple- 

 ment of the other, or what the other 

 wants to make it white light, that is, 

 the primitive and the accidental colour 

 when mixed together will make white 

 light. Hence the accidental colours 



