( 254) 
C, Aug. 14th) 15th, 16th, 17th, 18. time of exposure 40 hours; 
two tubes used, one of which worked 10 hours and was pretty 
hard, the other a very good tube which remained soft for the 
remainder of the time; developed in 10 minutes with glycine 
1 406: 
There is scarcely any fog on the plates. 
In order to enquire how wide that part of the diffraction slit was, 
through which the rays have. passed that have worked upon the 
photographic plate at a definite point, small round holes were drilled 
just as in our preceding investigation in one of the sides of the second 
slit and close to it at the extreme ends and in the centre. On 
account of this on the plates N°. 1 and N°. 2 serving for the 
measurement of the second slit, circular images had appeared and 
elongated ones on the plates A, B and C. (From these. pinhole- 
photographs of the active part of the anticathode, limited by the 
width of the first slit, is proved that this active part was only 
2mm. high). The distances between the centres of these images 
were divided by the dividing-machine into the same number of 
equal parts, so that the corresponding division-marks point to corre- 
sponding places of slit and image. 
For the measurement of N°. 1 and N° 2 object-elass D and 
measuring-eye-piece 2 were used where one division of the micrometer 
corresponds to 3,6 u. 
For the measurement of the image of the slit on A, B and (the 
mieroplanar I*, 2 was used as object-glass and as eye-piece the com- 
pensation eye-piece 6; one division of the eve-piece-mierometer corre- 
sponds to 55 gu, the magnifying power was 27 with a distance of the 
image of 25 cm. 
In the following table are mentioned for the successive division- 
marks indicated by their number in the first column: 
in column 2: the mean of the values derived from N°. 1 and 
N°. 2 of the width of the second slit in micra: 
in column 3: the double width of the second slit augmented by 
the width of the first slit (15 4), thus the theoretic width of the image, 
without diffraction ; the distanee between the photographie plate 
and the first slit being double the distance between the first and 
second slit; 
in column 4, 5 and 6: the width of the images respectively on 
A, B and Cas measured in divisions of the eye-piece-micrometer 
(div == 55); 
in column 7: the mean width of the images in miera (rounded off). 
EZ 
Proceedings Royal Acad, Amsterdam. Vol. V. 
