PASSAGE OF THE UNDEVIATED AND DEVIATED WAVES 35 



optical system in the manner indicated by the unbroken rays. These 

 rays of the zero spectral order converge upon the point C to form an 

 image of point C within the conjugate area of the diffraction plate. 

 Thereafter the undeviated rays diverge and spread with a high degree of 

 uniformity over the plane of the image. A second portion of the bundle 

 of rays which is incident upon the grating becomes deviated by diffrac- 

 tion as a set of rays which belong to the first spectral order. One of the 

 families of rays which belong to the first order is deviated downward 

 with respect to the undeviated rays of the zero order; the second of the 

 two families is deviated upward. The upward family of rays is focused 

 about the point C+i'; the downward family is focused about the 

 point C_i'. Points C+i' and (?_/ fall in the plane of the diffraction 

 plate and constitute secondary images of the point C as formed by the 

 set of rays belonging to the first spectral order. In a similar manner 

 the rays belonging to any higher spectral order n form secondary 

 images of C at points Cj^n ■ Only a limited number of the secondary 

 images C^J fall within the clear aperture of the diffraction plate. For 

 example, C+2' is illustrated as blocked by a limiting diaphragm which 

 restricts the clear aperture of the objective. Whenever a secondary 

 image produced by any spectral order does not fall within the clear 

 aperture of the objective, the corresponding portion of this order is 

 blocked and cannot participate in the image formation of the object 

 specimen. 



In the illustration of Fig. II. 9 only the secondary images C+i', C_i', 

 €-2 , and C_z' can participate with the primary image C" to determine 

 by interference the image formed in the plane of the image. In Fig. II. 9 

 not even the secondary image Cj^i' is complete, for the first-order ray 

 emanating from point h cannot reach point C+i'. The primary and 

 secondary images can be observed in a microscope by placing at the 

 plane of the condenser diaphragm an opaque disk which contains a 

 small opening at point C and by looking down the body tube of the 

 microscope whose eyepiece has been removed. If desired, the plane of 

 the diffraction plate may be examined with the auxiliary telescope which 

 is ordinarily furnished for the purpose of aligning the optical system of 

 the phase microscope. If the system has been aligned properly, the 

 bright primary image C of the zero order will fall well within the 

 conjugate area. When white light is used, the secondary images will be 

 spread into a spectrum because rays of longer wavelength suffer the 

 greater deviation at the object grating. Now the light carried by the 

 first and higher spectral orders belongs to the deviated wave. Hence 

 we see from the above considerations and experiments that in general 



