Ixxxviii 



INTRODUCTION TO OPTICS. 



us, the sun has quitted the spot he occupied on their departure ; yet we 

 see him in the direction of those rays, and consequently in a situation 

 which he had abandoned eight minutes and a half before. In speaking 

 of the sun's motion, we mean his apparent motion, produced by the diurnal 

 rotation of the earth, for the effect being the same whether it be our 

 earth or the heavenly bodies which move, it is more easy to represent 

 things as they appear to be, than as they really are. The refraction of 

 the sun's rays by the atmosphere prolongs our days, as it occasions our 

 seeing an image of the sun, both before he rises and after he sets ; for 

 below the horizon he still shines upon the atmosphere, and his rays are 

 thence refracted to the earth. So likewise we see an image of the sun 

 before he rises, the rays that previously fall upon the atmosphere being 

 reflected to the earth. 



In passing through a pane of glass the rays suffer two refractions, 

 which being in contrary directions, produce nearly the same effect as if no 

 refraction had taken place. 



Fig. 22, A A represents a thick pane of glass seen edgeways. When 

 the ray B approaches the glass at C, it is refracted by it ; and, instead of 

 continuing its course in the same direction, as the dotted line describes, 



Fig. 22. 



Fig. 23. 



.A 



it passes through the pane to D ; at that point returning into the air, it is 

 again refracted by the glass, but in a contrary direction, and in conse- 

 quence proceeds to E. Now the ray B C and the ray D E being parallel, 

 the light does not appear to have suffered any refraction ; for if a ray of 

 light passes from one medium into another, and through that into the first 

 again, the two refractions being equal and in opposite directions, no 

 sensible effect is produced ; for the direction is the same, and the little 

 space by which the ray is thrown to one side, as represented in fig. 22, is 

 necessarily less than the thickness of the medium, and the thickness of a 

 pane of glass is too little to be worth considering. But this is the case 

 only when the two surfaces of the refracting medium are parallel to each 

 other; if they are not, the two refractions may be made in the same 

 direction. Thus, when parallel rays (fig. 23) fall on a piece of glass hav- 

 ing a double convex surface, which is called a lens, that only which falls in 

 the direction of the axis of the lens is perpendicular to the surface ; the 

 other rays falling obliquely are refracted towards the axis, and will meet 

 at a point beyond the lens, called its focus. Of the three rays A, B,C, 

 which fall on the lens D E, the rays A and C are refracted in their 

 passage through it, to a and c, and on quitting the lens they undergo a 

 second refraction in the same direction, which unites them with the ray B, 



