Theories of Microscopical Vision. By A, E. Conrady. 543 



The second case to which we will pay attention will be a 

 dot-pattern arranged in equilateral triangles, as shown in fig. 126. 

 Similar reasoning to that just applied to the square pattern°shows 

 us three directions, a, b, c, in which the dots arrange themselves 

 into rows with a maximum distance apart ; next three directions, 

 d, e,f, of closer rows of dots ; and so on, the result being again that 

 a set of diffraction-spectra, as illustrated in fig. 126a, is formed 

 similar in arrangement to the pattern itself. 



The reasoning here used is directly applicable only to very 

 small perforations which can be considered as parts of separate and 

 distinct lines, a, b, c, d, etc. But as the diffraction-spectra are 



Fig. 123. 



Fig. 124. 



formed by light from all the perforations meeting with differences 

 of phase expressed by a whole number of wave-lengths, and as this 

 phase-relation will not be disturbed if all the perforations are 

 uniformly increased in size, it will be seen that the arrangement 

 of the diffraction-spectra must remain the same no matter how 

 large the perforation may become ; for that arrangement is deter- 

 mined by the configuration of similarly situated points in the 

 individual perforations. 



The relative brightness of, and the phase-relation between the 

 direct light and the different spectra will, however, depend upon 

 the relative size and upon the shape of the dots or perforations ; 



2 2 



