564 



For a plate parallel to {OOlj: 



in the direction of the ^-axis at 89.25 in.ni. from C 

 „ „ „ „ „ j5-axis „ 99.92 „ „ „ 



If now we calculate fi'om the measured distances of the «pots on 

 the photographic plate of nnhydrite, using A = 50 ra.m., the values 

 of k/2if, then ff, and finally 100 fg </ , — we shall find that all these 

 calculated values are a little smaller than the corresponding rational 

 parts of the above mentioned fundamental distances in the directions 

 of the resp. axes: moreover, these calculated values all appear to 

 be diminished in about the same ratio. 



In the case of the plioto parallel to jIOOj e.g. we measured for 

 some spots : 



in the direction of the 6'-axis : 31.2 mm.; 24.1 mm. and 19.9 mm. 

 from the centre; 



in the direction of the ö-axis : 27.4 mm.; and 21.8 mm. 



For the same points the calculated distances are: 



in the direction of the 6'-axis : 32 mm.; 24.8 mm.; and 22.4 mm. 



in the direction of the ^-axis: 28 mm.; and 22.4 mm. 



But calculations and measurements now appear in much better 

 agreement with each other, if we suppose A during the experiment 

 to have had a smaller value, and to have been about 48,3 mm. 



Now the thickness d of the cryslalplate was in our case precisely 

 1,64 mm. ; and thus we must conclude, that in this case we must 

 use in our calculations of the angles r/, for A not the value of the 

 distance from the front face of the crystal-plate to the piioto- 

 graphic film, but that from the hacliranl-ÏM-e of it to the photo- 

 graphic plate. 



In other analogous cases we indeed now learned, that if .4 was 

 supposed to be = 50 mm., during the experiment, the distances 

 from the projected spots to the centre C appeared to be always 

 too small, if in the final projection the distance OP is always kept 

 = 100 mm.; but that ordinarily a sufficient agreement between 

 calculation and experiment would result; if .1 during the experiment 

 is supposed to be (50 — y d)> where (/is the thickness of the crystal- 

 plate used. 



This influence of the plate-thickness becomes yet more evident, if 

 of the same crystal RöNTGiN-patterns are obtained with plates of 

 \ery different thickness ; for in that case the photos must manifest 

 different distances from 6' for the same spots. We wei-e able to 

 observe something of the kind in the measurement of two analogous 



