1187 
The preceding table gives the corresponding values of 7 for different 
a 
values of a 
§ 2. Instead of a single di-atomic gas molecule now a whole gas 
mass is irradiated; the dimensions of the irradiated quantity, however, 
will be comparatively small compared with the distance between the 
gas and the photographic plate Z (e.g. 1 mm. to 5 em.) 
We state '): 
The dark rings on the photographic plate will then — except for 
a slight diminution in sharpness — continue to be represented by 
equation (1). 
The decrease of sharpness corresponds with the slight changes in 
situation and size, which the rings (1) undergo when the centre of 
the molecule discussed in §1 is made to pass consecutively through 
all the points of the small irradiated region. 
§ 3. In the experimental realisation of these rings we are confronted 
with difficulties which are indeed very great, but yet possibly not 
insurmountable. 
1. The probably very slight intensity of the whole secondary 
radiation. In any case we shall choose vapours the atoms of which 
will be as heavy as possible’). 
2. The incident radiation must be as homogeneous as possible or 
anyway possess such a distribution of intensity in the spectrum that 
at least the first ring does not fade away entirely. In order to be 
still able to calculate a in the latter case from the distribution of 
light and dark, the spectrum distribution must be known from 
interference figures for crystals. 
3. If the first ring is to fall on favourable values of ¢, « : 4 must 
certainly be greater than one (see the table in § 1). 
4. Possibly the dark rings that originate from the amorphous 
glass vessel in which the vapour is contained, might be troublesome *). 
Then we should have to replace the amorphous glass e.g. by mica. 
APPENDIX. 
The secondary waves which two atoms A, B send to a definite 
1) See appendix. 
*) Or perhaps solutions; but here the phenomena are theoretically more com- 
plicated. 
5) They have not occurred in FRIEDRICH’s experiments with yellow wax. 
