Burton. — On the Minumini Visible in the Microscope. 251 



perpendicularly on two elements of a grating wLich are represented 

 in transverse section by the thick short lines c d. The lines //' re- 



/ 



/ 

























f 



n 



' 













■^^l 







7^^^^^^^^ 







~~-->, 



^ 



If 



"\\ 



9 



'i 







;^^ 









1 





Diagram (1). 



present the principal focal planes of the objective employed, and e e' 

 the infinitely thin lenses vrhich would produce refractions of the 

 transmitted Hght equivalent to those actually caused by the objective 

 itself ; p is the point at which the light transmitted directly thi'ough 

 the grating is brought to a focus. A bright line is consequently 

 produced there if the source of light be a slit. But there will also be 

 lateral diffracted rays, one of which is shown pursuing its course to 

 a focal point m q. li a i, the diiference in the length of the two 

 sides of the beam, be equal to a whole wave length, the two sides will 

 reach q in the same phase, and the result at q will be a bright line. 

 If the difference in length of path be equal to only half a wave length, 

 the result at q will be a dark line. In general, where the difference 

 in length of path equals an even multiple of a half wave length, a 

 bright line will be found in the plane of /; when, on the other hand, 

 the difference equals an uneven multiple of the wave length, the result 

 will be darkness. 



Let a be the angle of inclination of the diffracted ray, A its length 

 of wave, and a b the breadth of the successive intervals of the grating, 

 then we have for the first bright line sin a = A ^ ff 3 ; for the second 

 bright line, &m a = 2\ ^ a l, and so on. If Kght of a higher refran- 

 gibility, i. e., of shorter wave length, be used, q and q' will be fo.und 

 nearer to p than before, because a i will have been climinished, and 

 thus, if white light be employed, the image of the som^ce formed at q 

 and (f wiU be impure, and wiU appear fringed with colour, blue at 

 the inner, and red at the outer edges. 



If also the source of light be of sensible magnitude, the rays of 

 each colour will form an infinite number of apposed images of the 

 elements of its surface in and near q and q\ composing thereby per- 

 fect images which will overlay one another, and appear at each side 



E. I. A. PROC, SER. II., VOL. III. — SCrENCE. U 



