678 
beyond all doubt, that such a selective diminution of the 
intensities of some spots with respect to others really happened 
in cases where the ROnTGEN-rays had to pass through thicker layers 
of a crystalline medium. Undoubtedly this selective absorption is, at 
least partially, responsible for the abnormal intensity-relations in the 
composed photogram of the mica-piles, compared with the corre- 
sponding relations in the film-combination. However, a certain 
momentum for this appears also to be the strong veil on the back- 
ground of the photographic plate in the first case; a veil, which may 
in the final photos of the piles also be the cause of the absence 
of the outer and feebler spots of the image obtained with a single 
lamella, because the photographic plate could not be developed a 
sufficiently long time to make them appear upon it. This photographic 
veil is, therefore, also one of the causes of the misleading aspect 
of the photograms of the mica piles, so that they seem to be different 
from a true superposition of the images obtained with a single lamella. 
That besides this, also the use of the stereographical projections in- 
stead of the film-combination, formerly led us to a conclusion 
which is now acknowledged as erroneous, need not surprise us: 
for in the stereographical projections the intensity of the different 
spots was not measured photometrically, but estimated in a purely 
subjective manner, and in thus comparing different stereographical 
images with each other, properly incomparable intensities are checked 
with respect to each other. These circumstances may elucidate why 
the photographical images of the mica-piles were formerly not 
recognised as being mere superpositions of the single images com- 
posing them. However, the veil of the photographic plates is probably 
amongst all cooperating causes of greater influence than the unequal 
diminution of the intensities of the spots by selective absorption. In 
any case no truly new phenomenon is here present of a kind in- 
conceivable with respect to the generally adopted interpretation of 
diffraction-phenomena in crystals. 
Physical and Physico-chemical Laboratories of the Unwersity. 
Groningen, October 1920. 
