200 



Note. 



valve (or, more generally expressed, parallel to any of the three ordinary 

 sets of lines on it) : while rectangular fields, separated by dark broad 



beams, alternating from row to row, and these learns proceeding, as pale- 

 shadows, through the ivhite fields of the neighhouriug rows. If h in the 

 figure is the distance of the ordinary lines on P. angulatum, the interval a 

 between the consecutive beams is 



« : ^» = 2-2 : VS 



This unusual image results from the entering of a new set of diffrac- 

 tion pencils, which by a smaller aperture are shut off. The diffraction 

 spectre of any structure the elements of which are arranged in con- 

 gruent rows, crossing under angles of 120°, must have the following 

 form : — where the marks • denote the six spectres next to the image A of 



-0 



Fig. 3. 



the undiffracted pencil, Q the twelve spectres of a more distant circle. 

 Now, in the case of Pleurosigma to an aperture of moderate amount 



