INTERFERENCE OF DIRECT WITH REFLECTED LIGHT. 



243 



the inner margin consequently appears bluish, and the outer one 

 yellowish red. For the other maxima, of course, a less noticeable 

 superposition of the different colours takes place ; the combined 

 effect may be determined by reflected light, according to simple 

 rules, precisely as with Fresnel's mirror, or Newton's rings, upon 

 which points further details may be found in the text-books of 

 Physics. Under the Microscope the resulting colours appear 

 unaltered even for the first bright line if the objective is achro- 

 matic or over-corrected, whilst an under-corrected objective 

 inverts the order of the colours or lessens their intensity. No 

 special proof is therefore needed that 

 this influence of chromatic aberration 

 must affect the other interference lines 6 a 

 also. 



If the reflecting surface is spherically 

 curved, the angle <j>, which the reflected 

 rays make with the reflecting plane 

 or, what is equivalent, with the cor- 

 responding tangential plane has a 

 greater value for every successive inter- 

 ference line. In Fig. 133, A A is the 

 reflecting plane corresponding to the 

 point a, and B B the one corresponding 

 to the point 6; and 0' are the re- 

 spective angles of inclination of the 

 incident rays. Hence, the distances // 

 of the dark lines become less, accord- f ---;-* 

 ing to their proximity to the reflecting 

 surface. This is observed in air- 

 bubbles, mercury-globules, ami other reflecting bodies and with 

 special distinctness if, instead of the usual circular diaphragm, a 

 slit-shaped one is moved slowly backward and forward at a suit- 

 able distance from the object, to discover the most favourable, 

 position in relation to the reflecting surfaces. 



In like manner the lines which are seen here and there on the 

 walls in sections of tissue (e.g., cork) may be very well shown. 

 The increase or decrease of their distances, according to the 

 inclination of the incident rays, proves that they belong to the 

 above-described interference phenomena. 



FIG. 133. 



