230 



Transactions of the Society. 



ing points. The image of P. angulatum, with straight [direct] light, 

 by an immersion lens, is formed by seven pencils. 



As to Pleurosigma, whether there are three sets of lines or two, 

 the following will state my opinion. 



Fi<V 1. 



Abbe's Le:tt£R-T>e.c. /5- 187^. 



o <3 o o 



Fi^.a. 





F(^.5 



(-jiK-OCo) 



rx^ • • • • • «ri • • 



'<±^ 



F/6. f 



The diffraction-phenomena, produced by three sets, crossing at 

 60°, or by two sets, or by isolated apertures of any form, arranged 

 [as in fig. laj, are not different one from another in the first row 

 of spectra around the direct pencil. Those diffraction-phenomena 

 are different only in the more distant spectra, which differ in 

 position and relative brightness. 



