relative to the Interference of Light. 4S5 



immediately from the liquid, the stripes will be seen as formed 

 at its focus within the liquid. This will be immediately un- 

 derstood on I'ecollectinfT that the stripes seen by a lens, under 

 any circumstances, are those formed at its focus, and are to be 

 regarded simply as an optical image situated there. 



Hence also, if the lens have not its surface in contact witli 

 the liquid, but be placed at any distance from the surface less 

 than its focal length, it will still exhibit the stripes as formed 

 within the liquid. 



In this way the stripes will be seen narrower, or having less 

 values off than in air : and from the equation, 



c = -— cot 6, or A = 2c tan fl. 



it follows, that since 9 remains the same, X must be diminish- 

 ed within the medium. If the obtuse prism be employed, or 

 the interference be produced by reti'action, it will be maniiest 

 that, owing to the less difference in refractive power between 

 the glass and the liquid, than between the glass and air, the 

 rays will cross at a smaller augle; and if A remain the same, c 

 ought therefore to be greater; but in point of fact the stripes 

 are observed unaltered, or c remains the same: hence it fol- 

 lows that A must really be diminished. The diminution of 9 

 is evidently due to the difference of refractive power between 

 the liquid and air ; and if m be the relative index (calling the 

 angle in the liquid fl), we have sin fl = m sin 9; and since in 

 such very small angles we may substitute the sine for the tan- 

 gent, the formula gives, in air, A = 2c sin 9 



in the liquid, A, = 2c — sin 9, 

 ' ' m 



Or A, is a value diminished exactly in the ratio of the refractive 

 powers. Either of these experiments is, however, very trou- 

 blesome to manage ; and it is very difficult to compare satis- 

 factorily the breadth of the stripes in the two cases. The 

 following is much easier, and has the additional advantage of 

 being applicable to solid transparent bodies as well as to 

 liquids. ^ 



If a mass of a solid transparent body with plane parallel 

 surfaces, or of a fluid bounded by parallel plates of glass, be 

 simply placed between the prism or reflector and the eye- 

 lens, the latter having its focus within the medium as before, 

 then the rays on entering the medium, as they emerge from 

 the ])rism inclined to each other at an angle 29, that angle 

 will now be diminished to2 9;, and hence, as l)efore, the stripes 

 formed within the medium ought to become wider; l)ut in 

 point of fact they are seen unaltered: they may in tliis case be 

 3 K 2 -•"- ■ 



