3G4 



B. T. LOWNE ON INTERFERENCE PHENOMENA. 



interference images. Such test objects as Noberts' plates and fine 

 lines ruled on silvered glass are especially adapted for the investi- 

 gation of the nature of such images, which are usually, though not 

 perhaps invariably, false images. 



Fig. 11. 



If we examine Noberts' test-plate with the microscope by a 

 narrow pencil of transmitted light the lines will produce bands of 

 alternate light and darkness, crossing the objective, but there will 

 be no real loss of light, because the bright bands are twice as 

 bright as they would be if the light fell all over the objective, and 

 the dark bands are the same breadth as the bright bands. 



As the surface of the objective is indifferent, one part, theoreti- 

 cally at least, having the same value as another, the kind of inter- 

 ference I have described has no effect on the ordinary image. 



III. This Formation of Images and Diffraction Spectra. 



I must now ask you to remember the manner in which a micro- 

 scope acts. When you see an object clearly, it is in focus ; if you 

 remove the eye-piece, the relation of the object to the objective is 

 this : — The object lies in the anterior conjugate focus (Fig. 12, o), 

 which corresponds to a posterior conjugate focus, in the plane of 

 the eye-glass (e) ; that is, the magnified image of the object lies 

 exactly at the same level as the eye-glass occupies when in position. 



Now, the diaphragm of the condenser, w, by which your object is 

 illuminated, is further from the object-glass than the object, there- 

 fore its conjugate focus is nearer to the object-glass than the 

 eye-piece, for example, at h ; in general its image is just above 

 the object-glass. When a grating like Noberts' test-plate is 

 between the object-glass and the source of light, if you look down 

 the tube, the eye-piece having been removed, you see a central 



